vp8: correct function return types
vp8: correct function return types

left_above_mv and above_block_mv return as_int

as_int is defined as uint32_t in vp8/common/mv.h

Cleans up -Wextra warnings:
signed and unsigned type in conditional expression
this_mv->as_int = col ? d[-1].bmi.mv.as_int : left_block_mv(mic, i);
                                            ^
this_mv->as_int = row ? d[-4].bmi.mv.as_int : above_block_mv(mic, i, mis);
                                            ^
left_mv.as_int = col ? d[-1].bmi.mv.as_int :
                                           ^

Change-Id: Ia043764e4ce93d2152d2269b1c7b28b5d5f814cf

GrokImageCompression/aom,webmproject/libvpx,Topopiccione/libvpx,mbebenita/aom,mbebenita/aom,luctrudeau/aom,webmproject/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,smarter/aom,smarter/aom,smarter/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,mbebenita/aom,mbebenita/aom,luctrudeau/aom,webmproject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,GrokImageCompression/aom,mwgoldsmith/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,smarter/aom,luctrudeau/aom,mwgoldsmith/vpx,mbebenita/aom,Topopiccione/libvpx,mbebenita/aom,mbebenita/aom,mwgoldsmith/vpx,luctrudeau/aom,mbebenita/aom,webmproject/libvpx,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,luctrudeau/aom,webmproject/libvpx,smarter/aom,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,mwgoldsmith/vpx,Topopiccione/libvpx,GrokImageCompression/aom,mbebenita/aom,ShiftMediaProject/libvpx,GrokImageCompression/aom