ACPICA: Rename variable to match upstream

There is a variable name mismatch in acpi_ut_strtoul_multiply64()
between the ACPICA code in the kernel and the corresponding upstream
code which may be problematic if changes to this particular piece of
code are made upstream and ported to Linux, so rename the variable
in question to match its name in the upstream code.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>