media: i2c: imx290: Correct register sizes

Upstream commit 454a86f3

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Define registers with the appropriate size, using the variable-size
register access mechanism that has just been introduced. This simplifies
the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>