v4l2videoenc: fix a typo

in caps_merge I forget to remove comma.
gstv4l2allocator: align define of ALLOCATOR_FLAG

In most of place it using GST_V4L2_ALLOCATOR_FLAG_MMAP_REQBUF,
but there is some place using GST_V4L2_ALLOCATOR_FLAG_MMAP_REQBUFS,
I think GST_V4L2_ALLOCATOR_FLAG_MMAP_REQBUFS is better.