V4L/DVB (11841): core: fix potential mutex_unlock without mutex_lock in dvb_dvr_read

dvb_dvr_read may unlock the dmxdev mutex and return -ENODEV, except this
function is a file op and will never be called with the mutex held.

There's existing mutex_lock and mutex_unlock around the actual read but
it's commented out.  These should probably be uncommented but the read
blocks and this could block another non-blocking reader on the mutex
instead.

This change comments out the extra mutex_unlock.

[akpm@linux-foundation.org: cleanups, simplification]
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>