iwlwifi: mvm: fix locking in iwl_mvm_bt_rssi_event()

This will deadlock due to commit 9f34783863bea806
("iwlwifi: mvm: Implement BT coex notifications"):

  =============================================
  [ INFO: possible recursive locking detected ]
  3.5.0 #10 Tainted: G        W  O
  ---------------------------------------------
  kworker/2:1/5214 is trying to acquire lock:
   (&mvm->mutex){+.+.+.}, at: [<ffffffffa03be23e>] iwl_mvm_bt_rssi_event+0x5e/0x210 [iwlmvm]

  but task is already holding lock:
   (&mvm->mutex){+.+.+.}, at: [<ffffffffa03ab2d9>] iwl_mvm_async_handlers_wk+0x49/0x120 [iwlmvm]

  other info that might help us debug this:
   Possible unsafe locking scenario:

         CPU0
         ----
    lock(&mvm->mutex);
    lock(&mvm->mutex);

   *** DEADLOCK ***

Change-Id: I9104f252b34676e2f7ffcd51166f95367e08a4d9
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-on: https://gerrit.rds.intel.com/21887


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

Conflicts:
	drivers/net/wireless/iwlwifi/mvm/bt-coex.c