[media] DVB: dvb_frontend: update locking in dvb_frontend_{add, get_event}

  - fepriv->parameters_out isn't protected by events->mtx, so
    move the call to fe->ops.get_frontend out of the locked area.
  - move the assignment of e->status into the locked area.

  - use direct assignment instead of memcpy.

  - use mutex_lock instead of mutex_lock_interruptible, because
    all code paths protected by this mutex won't block.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>