V4L/DVB (5933): Dvb-usb/af9005-fe.c: error check fixes

This patch:
- adds a missing error check and
- removes an error check that could never be true

Both spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Luca Olivetti <luca@ventoso.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>