mtd: devices: mchp48l640: Fix memory leak on cmd

The allocation for cmd is not being kfree'd on the return leading to
a memory leak. Fix this by kfree'ing it.

Addresses-Coverity: ("Resource leak")
Fixes: 88d12502 ("mtd: devices: add support for microchip 48l640 EERAM")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210712145214.101377-1-colin.king@canonical.com