ARM: mach-shmobile: Kill off unused !gpio_is_valid() case

The Card Detect GPIOs used on AP4EVB and Mackerel are
alwayws valid, so kill off the unused !gpio_is_valid()
case.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>