V4L/DVB: cx231xx: cleanup dvb_attach() return value handling

Remove the following sparse error (see "make C=1"):
 * error: incompatible types for operation (<)
       left side has type struct dvb_frontend *
       right side has type int

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>