mtd: cfi_cmdset_0002: fix crash when erasing/writing AMD cards

Erasing an AMD linear flash card (AM29F016D) crashes after the first
sector has been erased. Likewise, writing to it crashes after two bytes
have been written. The reason is a missing check for a null pointer -
the cmdset_priv field is not set for this type of card.

Fixes: 4844ef80 ("mtd: cfi_cmdset_0002: Add support for polling status register")
Signed-off-by: Andreas Persson <andreasp56@outlook.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/DB6P189MB05830B3530B8087476C5CFE4C1159@DB6P189MB0583.EURP189.PROD.OUTLOOK.COM