iwlwifi: mvm: fix Rx with both chains

commit 5c90422439d6
"iwlwifi: mvm: don't allow diversity if BT Coex / TT forbid it"
broke Rx with 2 chains for diversity.
This had an impact on throughput where we're using only a single
stream (11a/b/g APs, single stream APs, static SMPS).

Fixes: 5c90422439d6 ("iwlwifi: mvm: don't allow diversity if BT Coex / TT forbid it")
Cc: Stable <stable@vger.kernel.org>  [3.16+]
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>