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>