pch_uart: Fix missing break for 16 byte fifo

Otherwise we fall back to the wrong value.

Reported-by:  <dcb314@hotmail.com>
Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=44091


Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>