iwlwifi: mvm: don't use SAR Geo if basic SAR is not used

We can't use SAR Geo if basic SAR is not enabled, since the SAR Geo
tables define offsets in relation to the basic SAR table in use.

To fix this, make iwl_mvm_sar_init() return one in case WRDS is not
available, so we can skip reading WGDS entirely.

Fixes: a6bff3cb ("iwlwifi: mvm: add GEO_TX_POWER_LIMIT cmd for geographic tx power table")
Cc: stable@vger.kernel.org # 4.12+
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>