iwlwifi: mvm: take mutex for calling iwl_mvm_get_sync_time()

We need to take the mutex to call iwl_mvm_get_sync_time(), do it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210115130252.4bb5ccf881a6.I62973cbb081e80aa5b0447a5c3b9c3251a65cf6b@changeid