[media] v4l2-ioctl: fix W=1 warnings

Since the prt_names() macro is always called with an unsigned index the
((a) >= 0) condition is always true and gives a compiler warning when
compiling with W=1.
Rewrite the macro to avoid that warning, but cast the index to unsigned
just in case it is ever called with a signed argument.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>