lfocontrolsource: fix clang compiler warning
lfocontrolsource: fix clang compiler warning

Cast enum to int before checking for negative values, which are
impossible according to the enum list.

gstlfocontrolsource.c:652:45: error: comparison of unsigned enum expression < 0
      is always false [-Werror,-Wtautological-compare]
  if (waveform >= num_waveforms || waveform < 0) {
                                   ~~~~~~~~ ^ ~

https://bugzilla.gnome.org/show_bug.cgi?id=653137

lovebug356/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,surround-io/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer,krichter722/gstreamer,ylatuya/gstreamer,collects/gstreamer,ensonic/gstreamer,magcius/gstreamer,ylatuya/gstreamer,magcius/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,magcius/gstreamer,cablelabs/gstreamer,centricular/gstreamer,centricular/gstreamer,collects/gstreamer,jpxiong/gstreamer,drothlis/gstreamer,ensonic/gstreamer,Distrotech/gstreamer,krichter722/gstreamer,Distrotech/gstreamer,ensonic/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,jpxiong/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,mparis/gstreamer,magcius/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,krieger-od/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,surround-io/gstreamer,ylatuya/gstreamer,lovebug356/gstreamer,collects/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,cfoch/gstreamer,collects/gstreamer,cablelabs/gstreamer,centricular/gstreamer,lubosz/gstreamer,drothlis/gstreamer,lubosz/gstreamer,ylatuya/gstreamer,drothlis/gstreamer,justinjoy/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,drothlis/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,mparis/gstreamer,drothlis/gstreamer,cablelabs/gstreamer,Distrotech/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,lubosz/gstreamer,ylatuya/gstreamer,lovebug356/gstreamer,centricular/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,justinjoy/gstreamer,lubosz/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,Distrotech/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,ensonic/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,collects/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,lovebug356/gstreamer,Distrotech/gstreamer,cfoch/gstreamer,centricular/gstreamer,Lachann/gstreamer,magcius/gstreamer,justinjoy/gstreamer