[media] v4l2-dv-timings: replace hsync magic number with a macro

This change will not change timing calculation. In CVT generator
spreadsheet the nominal value of hsync (as a percentage of line)
is 8 percent.

Cc: Martin Bugge <marbugge@cisco.com>
Signed-off-by: Prashant Laddha <prladdha@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>