mtd: maps: bcm963xx-flash: clean up printk usage

Replace raw printk's with their pr_XXX equivalent and unify broken up
strings so they become grepable.

Also replace the PFX definition with a pr_fmt().

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>