mtd: rawnand: gpmi: remove double assignment to block_size

The variable block_size is being assigned to itself and to
geo->ecc_chunk_size.  Clean up the double assignment by removing
the assignment to itself.

Addresses-Coverity: ("Evaluation order violation")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>