x86, intel-mid: Return proper error code from get_gpio_by_name()

This patch cleans up get_gpio_by_name() to return an error code
instead of hardcoded -1.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Link: http://lkml.kernel.org/r/1389913624-9149-4-git-send-email-david.a.cohen@linux.intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>