powercap/drivers/dtpm: Fix size of object being allocated

The kzalloc allocation for dtpm_cpu is currently allocating the size
of the pointer and not the size of the structure. Fix this by using
the correct sizeof argument.

Addresses-Coverity: ("Wrong sizeof argument")
Fixes: 0e8f68d7 ("powercap/drivers/dtpm: Add CPU energy model based support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>