sys/v4l2/v4l2src_calls.c: Check whether the device supports setting the framerate before trying to set it and then po...

Original commit message from CVS:
Based on patch by: William M. Brack <wbrack at mmm com hk>
* sys/v4l2/v4l2src_calls.c: (fractions_are_equal),
(gst_v4l2src_set_capture):
Check whether the device supports setting the framerate before
trying to set it and then posting a warning or error if it doesn't
work (#516649, #520092). Also compare fractions more correctly.