media: ov5693: count num_supplies using array_size

Instead of hardcode OV5693_NUM_SUPPLIES in a define is better use
ARRAY_SIZE function to count the number of supplies from
ov5693_supply_names array

Signed-off-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>