gpio/ep93xx: fix pr_fmt output

With this driver now being moved from arch/arm/mach-ep93xx/gpio.c to
drivers/gpio/gpio-ep93xx.c the prefix "ep93xx" is now redundant in the
pr_* output. Using KBUILD_MODNAME is sufficient.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>