usb: isp1301-omap: mark power_up as __maybe_unused

The power_up function is used for otg or udc mode, but nost when
the driver is only configured for host mode:

drivers/usb/phy/phy-isp1301-omap.c:261:13: error: 'power_up' defined but not used [-Werror=unused-function]

This marks the function __maybe_unused to avoid the warning and
silently drop the definition when it is unused.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/phy/phy-isp1301-omap.c

@@ -258,7 +258,7 @@ static void power_down(struct isp1301 *isp)
 	isp1301_clear_bits(isp, ISP1301_MODE_CONTROL_1, MC1_DAT_SE0);
 }
 
-static void power_up(struct isp1301 *isp)
+static void __maybe_unused power_up(struct isp1301 *isp)
 {
 	// isp1301_clear_bits(isp, ISP1301_MODE_CONTROL_2, MC2_GLOBAL_PWR_DN);
 	isp1301_clear_bits(isp, ISP1301_MODE_CONTROL_1, MC1_SUSPEND);