mtd: *nftl: return -ENOMEM when kmalloc failed

The driver is using -1 instead of the -ENOMEM defined macro to
specify that a buffer allocation failed. Using the correct error
code is more intuitive

Smatch tool warning:
drivers/mtd/inftlmount.c:333 check_free_sectors() warn: returning -1
instead of -ENOMEM is sloppy
drivers/mtd/nftlmount.c:272 check_free_sectors() warn: returning -1
instead of -ENOMEM is sloppy

No functional change, just more standardized.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
[<miquel.raynal@bootlin.com>: Fixed the title]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1619429543-52234-1-git-send-email-yang.lee@linux.alibaba.com