sh: pfc: Release spinlock in sh_pfc_gpio_request_enable() error path

The sh_pfc_gpio_request_enable() function acquires a spinlock but fails
to release it before returning if the requested mux type is not
supported. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>