All implementations of the .pin_to_portcr() method implement the same
conversion from Port Control Register offset to virtual address.  Factor
it out into the two callers.
Remove the pfc parameter, as it is no longer used.

Note that the failure handling in r8a7740_pin_to_portcr() is pro forma,
as the function is never called with an invalid pin number.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/a485d4986a17259256988eb14e3a4c2b8d61c303.1640270559.git.geert+renesas@glider.be