sh-pfc: Replace pinmux_enum_id typedef with u16

The typedef only conceals the real variable type without bringing any
additional value (see Documentation/CodingStyle, section 5.b). Moreover,
it polutes the pinmux namespace. Replace it with the integer type it
used to hide.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Yusuke Goda <yusuke.goda.sx@renesas.com>