serial: sh-sci: Do not open-code sci_getreg()

Replace open-coded variants of sci_getreg() by function calls, and drop
intermediate variables where appropriate.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>