mtd: phram: use KBUILD_MODNAME

Use the more standard #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
No change in output strings.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>