mtd: mchp48l640: silence some uninitialized variable warnings

Smatch complains that zero length read/writes will lead to an
uninitalized return value.  I don't know if that's possible, but
it's nicer to return a zero literal anyway so let's do that.

Fixes: 88d12502 ("mtd: devices: add support for microchip 48l640 EERAM")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/YMyir961W28TX5dT@mwanda