serial: sa1100: make sa1100_register_uart_fns a function

Calling sa1100_register_uart_fns() leaves the port structure
unused when CONFIG_SERIAL_SA1100 is disabled, and we get a
compiler warning about that:

arch/arm/mach-sa1100/badge4.c:317:31: warning: 'badge4_port_fns' defined but not used [-Wunused-variable]
 static struct sa1100_port_fns badge4_port_fns __initdata = {

This turns the two empty macros into empty inline functions,
which has the same effect, but lets the compiler know that the
variables are intentionally unused.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>