m68k: mvme: Remove overdue #warnings in RTC handling

The warnings were introduced when converting the MVME147 and MVME16x
RTC handling from gettod to hwclk.  Replace the #warning by a comment,
and return an error to inform the upper layer that writing to the RTC is
not yet supported.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210907124511.2723414-1-geert@linux-m68k.org