media: wl128x: Fix some error handling in fmc_prepare()

The st_register() returns have changed over time, but these days it
never returns -1.  We should just be checking for any negative error
codes.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>