drm/i2c: tda998x: fix sync generation and calculation

This fixes the wrong sync generation and sync calculation of TDA998x
for HS/VS-based sync detection.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Russell King <rmk_kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>