V4L/DVB (8378): cx18: move cx18_av_vbi_setup to av-core.c and rename to cx18_av_std_setup

Same issue as for cx25840: this function sets up the standard timings
and has nothing to do with VBI setup.

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