media: et8ek8: Decrease stack usage

The et8ek8 driver combines I²C register writes to a single array that it
passes to i2c_transfer(). The maximum number of writes is 48 at once,
decrease it to 8 and make more transfers if needed.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>