[media] i2c: Improve a size determination

[media] i2c: Improve a size determination

Replace the specification of a data structure by pointer dereferences
as the parameter for the operator "sizeof" to make size
determination a bit safer according to the Linux coding style convention.

This issue was detected by using the Coccinelle software.

[mchehab@s-opensource.com: merged similar patches]
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>