[media] tvp5150: replace MEDIA_ENT_F_CONN_TEST by a control

MEDIA_ENT_F_CONN_TEST is not really a connector, it is actually
a signal generator. Also, as other drivers use the
V4L2_CID_TEST_PATTERN control for signal generators, let's change
the driver accordingly.

Tested with Terratec Grabster AV350.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>