V4L/DVB (5415): Msp_attach must return 0 if no msp3400 was found.

Returning -1 causes the probe to stop, but it should just continue
instead.

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