V4L/DVB: s2255drv: removal of big kernel lock

big kernel lock removed from open function.
v4l2 code does not require locking the open function except
to check asynchronous firmware load state, which is protected
by a mutex

Signed-off-by: Dean Anderson <dean@sensoray.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>