staging: mt7621-mmc: Fix printk() facility level

The patch fixes the following issue:
WARNING: printk() should include KERN_<LEVEL> facility level

Signed-off-by: Sergej Perschin <ser.perschin@gmail.com>
Signed-off-by: Marcel Budoj <marcel.budoj@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>