V4L/DVB: s2255drv: remove dead code

My concern initially was we dereference "dev" in the parameter list to
s2255_dev_err() but it turns out that code path is never used.
The s2255_stop_readpipe() is only called from one place and "dev" is
never null.  So this patch just removes the whole condition here.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: linux-dev@sensoray.com
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>