V4L/DVB (7978): cx18: explicitly test for XC2028 tuner

Testing whether xceive_pin is non-zero is not good enough as 0 is a valid
value. Instead explicitly test whether the Xceive tuner is used.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>