mtd: Add WARN_ON_ONCE() to mtd_read() to check the return value

If the driver cannot read all the requested data, -EBADMSG or
-EUCLEAN should never be returned.

Add a WARN_ON_ONCE() to help driver developers detect this error.

Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230926065733.3240322-1-wangzhaolong1@huawei.com