macintosh/via-pmu: Clean up interrupt statistics

Replace an open-coded ffs() with the function call.
Simplify an if-else cascade using a switch statement.
Correct a typo and an indentation issue.

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>