serial: UniPhier: use 32 bit register access

For PH1-Pro4, the 8 bit write access to LCR register (offset = 0x11)
is not working correctly.  As a side effect, it also modifies MCR
register (offset = 0x10) and results in unexpected behavior.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>