iwlwifi: mvm: Check the sta is not NULL in iwl_mvm_cfg_he_sta()

Fix a kernel panic by checking that the sta is not NULL.
This could happen during a reconfig flow, as mac80211 moves the sta
between all the states without really checking if the previous state was
successfully set. So, if for some reason we failed to add back the
station, subsequent calls to sta_state() callback will be done when the
station is NULL. This would result in a following panic:

BUG: unable to handle kernel NULL pointer dereference at
0000000000000040
IP: iwl_mvm_cfg_he_sta+0xfc/0x690 [iwlmvm]
[..]
Call Trace:
 iwl_mvm_mac_sta_state+0x629/0x6f0 [iwlmvm]
 drv_sta_state+0xf4/0x950 [mac80211]
 ieee80211_reconfig+0xa12/0x2180 [mac80211]
 ieee80211_restart_work+0xbb/0xe0 [mac80211]
 process_one_work+0x1e2/0x610
 worker_thread+0x4d/0x3e0
[..]

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>