staging: greybus: power_supply: replace kzalloc by kcalloc

According to checkpatch.pl, kcalloc should be preferred to kzalloc with
multiply.

Signed-off-by: JB Van Puyvelde <jbvanpuyvelde@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>