mtd: Do not allow MTD devices with inconsistent erase properties

When mtd->erasesize is 0 or mtd->_erase is NULL, that means the device
does not support the erase operation, which in turn means it should
have the MTD_NO_ERASE flag set.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Miquel Raynal <miquel.raynal@free-electrons.com>