serial: Use B0 instead of implicit zero assumption

Compare against B0 rather than assume B0 equals zero, which is true for
all archs but explicit B0 makes the code more obvious.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230309080923.11778-3-ilpo.jarvinen@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>