leds: just ignore invalid GPIOs in leds-gpio

Fix build problems with leds-gpio:

  CC      drivers/leds/leds-gpio.o
drivers/leds/leds-gpio.c: In function 'create_gpio_led':
drivers/leds/leds-gpio.c:85: warning: 'return' with no value, in function returning non-void

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>