serial: sh-sci: fix handling of SCIFB sh-mobile ports

SCIFB ports have a slightly different register layout and a different FIFO
size from SCIFA ports, in DMA mode they have to be treated just like SCIFA.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>