media: i2c: fix warning in Aptina MT9V111

This fixes the "'idx' may be used uninitialized in this function"
warning:

	drivers/media/i2c/mt9v111.c: In function 'mt9v111_set_format':
	drivers/media/i2c/mt9v111.c:887:15: warning: 'idx' may be used uninitialized in this function [-Wmaybe-uninitialized]
	  unsigned int idx;
	               ^~~

Signed-off-by: Jasmin Jessich <jasmin@anw.at>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>