serial: 8250: omap: Move uart_write() inside PM section

If CONFIG_PM is not set (e.g. m68k/allmodconfig):

    drivers/tty/serial/8250/8250_omap.c:169:13: error: ‘uart_write’ defined but not used [-Werror=unused-function]
      169 | static void uart_write(struct omap8250_priv *priv, u32 reg, u32 val)
	  |             ^~~~~~~~~~

Fix tis by moving uart_write() inside the existing section protected
by #ifdef CONFIG_PM.

Reported-by:  <noreply@ellerman.id.au>
Link: http://kisskb.ellerman.id.au/kisskb/buildresult/14925095/
Fixes: 398cecc2

 ("serial: 8250: omap: Fix imprecise external abort for omap_8250_pm()")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230515065706.1723477-1-geert@linux-m68k.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>