iwlwifi: mvm: don't lock mutex in RCU critical section

We cannot lock a mutex while we're in an RCU critical section. At
the same time, we're accessing data structures that are protected
by the mvm->mutex anyway, so just move the entire locking here to
use only that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210411124417.4d27bd36e10e.I1fd8e8fe442c41a5deaa560452b598ed7a60ada5@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>