mtd: mtd_oobtest: printk -> pr_{info,err,crit}

Use pr_info() and pr_err() while defining pr_fmt(). This saves a few
characters, joins a few lines, and makes the code a little more readable
(and grep-able).
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Vikram Narayanan <vikram186@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>