serial/omap: mark wait_for_xmitr as __maybe_unused

serial/omap: mark wait_for_xmitr as __maybe_unused

The wait_for_xmitr() function is only used if CONFIG_CONSOLE_POLL
or CONFIG_SERIAL_OMAP_CONSOLE are set, but when both are disabled,
the compiler warns about it being unused:

drivers/tty/serial/omap-serial.c:1168:13: warning: 'wait_for_xmitr' defined but not used [-Wunused-func

We could add more #ifdefs to work around it, but adding __maybe_unused
seems nicer.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 2172076d2399 ("serial/omap-serial: Deinline wait_for_xmitr, save 165 bytes")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>