serial: 8250_uniphier: fix dl_read and dl_write functions

The register offset must be shifted by regshift, otherwise the
baudrate is not set.  I missed the issue probably because the
divisor register was already set by the boot loader.

Fixes: 1a8d2903 ("serial: 8250_uniphier: add UniPhier serial driver")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>