sysreset: Fix unsupported request return values

The description of the sysreset request method in <sysreset.h> says that
the return value should be -EPROTONOSUPPORT if the requested reset type
is not supported by this device.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/sysreset/poweroff_gpio.c b/drivers/sysreset/poweroff_gpio.c
index a5c24fd..ad04e4b 100644
--- a/drivers/sysreset/poweroff_gpio.c
+++ b/drivers/sysreset/poweroff_gpio.c
@@ -33,7 +33,7 @@
 	int r;
 
 	if (type != SYSRESET_POWER_OFF)
-		return -ENOSYS;
+		return -EPROTONOSUPPORT;
 
 	debug("GPIO poweroff\n");