gpio: intel-mid: fix the incorrect return of idle callback

intel_gpio_runtime_idle should return correct error code if it do fail.
make it more correct even though -EBUSY is the most possible return value.

Signed-off-by: bo.he <bo.he@intel.com>
Signed-off-by: xinhui.pan <xinhuiX.pan@intel.com>
Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>