phy: twl4030: use __maybe_unused to hide pm functions

The twl4030 USB PHY driver uses UNIVERSAL_DEV_PM_OPS to access
its suspend/resume functions, which causes a warning about
unused symbols when CONFIG_PM is disabled:

drivers/phy/phy-twl4030-usb.c:394:12: error: 'twl4030_usb_runtime_suspend' defined but not used [-Werror=unused-function]
drivers/phy/phy-twl4030-usb.c:408:12: error: 'twl4030_usb_runtime_resume' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations to let the compiler know
it can silently drop the function definition.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>