Properly initialise vaddr pointers in cpc_read() and cpc_write() to
NULL instead of 0.

This fixes the following sparse warnings:
drivers/acpi/cppc_acpi.c:937:31: warning: Using plain integer as NULL pointer
drivers/acpi/cppc_acpi.c:982:31: warning: Using plain integer as NULL pointer

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Len Brown <lenb@kernel.org>