iwlwifi: mvm: don't check system_pm_mode without mutex held

iwlwifi: mvm: don't check system_pm_mode without mutex held

When we want to stop TX'ing because we are suspending, we
have two options: either we check system_pm_mode or we
check the mvm's status that has a bit for the suspend
flow.
The latter is better because test_bit is atomic. Also
add a call to synchronize_net after we set the bit to
make sure that all the new Tx see the bit before we
actually complete the suspend flow.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210117130510.243c88781302.I5c0379c5a7e5d49410569e7fcd2fff7a419c6dea@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>