mtd: mtdoops: change printk() to counterpart pr_ functions

To comply with latest kernel code requirement, change printk() to
counterpart pr_ functions in mtdoops driver:
- change printk(INFO) to pr_info()
- change printk(DEBUG) to pr_debug()
- change printk(WARNING) to pr_warn()
- change printk(ERR) to pr_err()

Note that only if dynamic debugging is enabled or DEBUG is defined,
printk(KERN_DEBUG) and pr_debug() are equivalent; Otherwise pr_debug()
is no-op, causing different behavior.

Signed-off-by: Ray Zhang <sgzhang@google.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221010045549.2221965-2-sgzhang@google.com