serial: sh-sci: Remove unused STEPFN() macro

The last user of the STEPFN() macro was removed in commit
d5cb1319 ("serial: sh-sci: Remove manual break debouncing").

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/970387d104dea5bb7ea674bb89229641467e629b.1623076891.git.geert+renesas@glider.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>