macintosh/via-pmu: Don't clear shift register interrupt flag twice

The shift register interrupt flag gets cleared in via_pmu_interrupt()
and once again in pmu_sr_intr(). Fix this theoretical race condition.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>