iwlwifi: mvm: fix kernel panic in case of assert during CSA

During CSA, we briefly nullify the phy context, in __iwl_mvm_unassign_vif_chanctx.
In case we have a FW assert right after it, it remains NULL though.
We end up running into endless loop due to mac80211 trying repeatedly to
move us to ASSOC state, and we keep returning -EINVAL. Later down the road
we hit a kernel panic.

Detect and avoid this endless loop.

Signed-off-by: Sara Sharon <sara.sharon@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.20201107104557.d64de2c17bff.Iedd0d2afa20a2aacba5259a5cae31cb3a119a4eb@changeid