serial: sb1250-duart: clean up after SIBYTE_BCM1x55 removal

With commit b984d7b5

 ("MIPS: sibyte: Remove unused config option
SIBYTE_BCM1x55"), an #if in the Broadcom SiByte DUART driver can be
simplified.

Simplify the prepreprocessor condition after config SIBYTE_BCM1x55 removal.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20230323071053.417-1-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>