ACPICA: Utilities: split hex detection into smaller functions

acpi_ut_implicit_strtoul64() called acpi_ut_detect_hex_prefix() and
ignored the return value. Instead, use acpi_ut_remove_hex_prefix().

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