[media] wl128x: fix compiler warning + wrong write() return

The fix is to check for ret and return -EFAULT if non-zero.

I also noticed that write() didn't return the number of bytes written.
Fixed as well.

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