iwlwifi: mvm: fix first_antenna

first_antenna is supposed to return the first antenna as a
0-based bitmap: ANT_A is BIT(0), ANT_B is BIT(1), etc...
Since ffs is 1 based (ffs(BIT(0)) = 1), then we had an
off-by-one bug:
BIT(ffs(ANT_A)) = BIT(ffs(BIT(0))) = BIT(1) = ANT_B.
So what we really want is:
BIT(ffs(ANT_A) - 1) = BIT(ffs(BIT(0)) - 1) = BIT(0) = ANT_A.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/iwlwifi/mvm/utils.c

@@ -253,8 +253,9 @@ int iwl_mvm_rx_fw_error(struct iwl_mvm *mvm, struct iwl_rx_cmd_buffer *rxb,
 u8 first_antenna(u8 mask)
 {
 	BUILD_BUG_ON(ANT_A != BIT(0)); /* using ffs is wrong if not */
-	WARN_ON_ONCE(!mask); /* ffs will return 0 if mask is zeroed */
-	return (u8)(BIT(ffs(mask)));
+	if (WARN_ON_ONCE(!mask)) /* ffs will return 0 if mask is zeroed */
+		return BIT(0);
+	return BIT(ffs(mask) - 1);
 }
 
 /*