dc1394: Fix invalid property value handling.
dc1394: Fix invalid property value handling.

Unlikely to be enoucntered, but error handling could have been being
skipped for an invalid value for framerate or video mode. (The
dc1394framerate_t and dc1394video_mode_t are enum types which can be
unsigned, so using -1 as an indicator of invalid value does not work.)

Pointed out by clang++ warnings.

git-svn-id: 03a8048b5ee8463be5048a3801110fb50f378627@12331 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd

kmdouglass/Micro-Manager,kmdouglass/Micro-Manager