[media] wl128x: fmdrv_tx: fix signedness bugs

Message-Id: <1322820073-19347-4-git-send-email-xi.wang@gmail.com>

The error handling with (ret < 0) didn't work where ret is a u32.
Use int instead.  To be consistent we also change the functions to
return an int.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>