[media] rtl28xxu: do not return error for unimplemented fe callback

Use of frontend callback is highly hardware design dependent
and whole callback could be optional in many cases. Returning
error by default when callback is not implemented is stupid.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>