mtd: mtd_nandecctest: improve message output

This includes the message related changes:

- Use pr_* instead of printk
- Print hexdump of ECC code if test fails
- Change log level for hexdump of data from KERN_DEBUG to KERN_INFO
- Factor out the hexdump code into a separate function

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>