ACPICA: Add missing type casts in GPE register access code

Type casts needed on 32-bit systems are missing in two places in the
GPE register access code, so add them.

Fixes: 7a8379eb ("ACPICA: Add support for using logical addresses of GPE blocks")
Reported-and-tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>