iwlwifi: acpi: generalize iwl_mvm_sar_find_wifi_pkg()

Move this function to acpi.c, renaming it to iwl_acpi_get_wifi_pkg(),
because it can also be used with other methods (i.e. SPLC and WRDD).

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>