V4L/DVB: firedtv: remove check for interrupting signal

FCP transactions as well as CMP transactions were serialized with
mutex_lock_interruptible.  It is extremely unlikly though that a signal
will arrive while a concurrent process holds the mutex.  And even if one
does, the duration of a transaction is reasonably short (1.2 seconds if
all retries time out, usually much shorter).

Hence simplify the code to plain mutex_lock.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>