sh: pfc: get_config_reg() shift clean up

Clean up the f_width shift code in get_config_reg().

Reported-by: Ryusuke Sakato <ryusuke.sakato.bx@renesas.com>
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>