V4L/DVB (3662): Don't set msp3400c-non-existent register

The driver tried to set a register that is not present on msp3400c devices.
Add the missing test.

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