mtd: parsers: bcm47xxpart: print correct offset on read error

mtd_read() gets called with offset + 0x8000 as argument so use the same
value in pr_err().

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220317114316.29827-1-zajec5@gmail.com