mtd: nand: change KERN_DEBUG to KERN_INFO

Soon we will change many printk statements into pr_* statements, i.e.,
'printk(KERN_INFO, ...)' becomes 'pr_info(...)'. However, this means that
KERN_DEBUG messages will become pr_debug() statements and therefore will
not be activated by default - they must be enabled using dynamic debug.
So, for important DEBUG messages, we will simply upgrade these to INFO
so that they appear by default.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>