drivers: macintosh: rack-meter: fix bogus memsets

Fix bogus memsets pointed out by sparse:

linux-v4.3/drivers/macintosh/rack-meter.c:157:15: warning: memset with byte count of 0
linux-v4.3/drivers/macintosh/rack-meter.c:158:15: warning: memset with byte count of 0

Probably "&" is mistyped "*"; use ARRAY_SIZE to make it more safe.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>