media: ov2680: Fix vflip / hflip set functions

ov2680_vflip_disable() / ov2680_hflip_disable() pass BIT(0) instead of
0 as value to ov2680_mod_reg().

While fixing this also:

1. Stop having separate enable/disable functions for hflip / vflip
2. Move the is_streaming check, which is unique to hflip / vflip
   into the ov2680_set_?flip() functions.

for a nice code cleanup.

Fixes: 3ee47cad ("media: ov2680: Add Omnivision OV2680 sensor driver")
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>