mtd: rawnand: omap2: drop unused variable

devsize is not used anywhere in code. Drop it.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211209090458.24830-7-rogerq@kernel.org