V4L/DVB (5965): Frontend_ioctl(): fix check-after-use

The Coverity checker spotted that we have already oops'ed if "fe" was NULL.

Since "fe" being NULL seems impossible at this point this patch removes
the NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>