We should zero an array using sizeof instead of number of elements.

Fixes the following compiler (GCC 7.3.0) warnings:

drivers/macintosh/rack-meter.c: In function 'rackmeter_do_pause':
drivers/macintosh/rack-meter.c:157:2: warning: 'memset' used with length equal to number of elements without multiplication by element size [-Wmemset-elt-size]
drivers/macintosh/rack-meter.c:158:2: warning: 'memset' used with length equal to number of elements without multiplication by element size [-Wmemset-elt-size]

Fixes: 4f7bef7a ("drivers: macintosh: rack-meter: fix bogus memsets")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>