iwlwifi: mvm: fix RCU splat in TKIP's update_key

The commit below mistakenly changed an rcu_dereference_check
to a rcu_dereference_protected which introduced the
following RCU warning:

[ INFO: suspicious RCU usage. ]
 4.6.0-rc7-next-20160513-dbg-00004-g8de8b92-dirty #655 Not tainted
 -------------------------------
 drivers/net/wireless/intel/iwlwifi/mvm/mvm.h:1069 suspicious rcu_dereference_protected() usage!
 Call Trace:
  [<ffffffff8106b836>] lockdep_rcu_suspicious+0xf7/0x100
  [<ffffffffa03b2321>] iwl_mvm_get_key_sta.part.0+0x5d/0x80 [iwlmvm]
  [<ffffffffa03b4acb>] iwl_mvm_update_tkip_key+0xd3/0x162 [iwlmvm]
  [<ffffffffa03a2b60>] iwl_mvm_mac_update_tkip_key+0x17/0x19 [iwlmvm]
  [<ffffffffa0329646>] ieee80211_tkip_decrypt_data+0x22c/0x24b [mac80211]
  [<ffffffffa0318bb1>] ieee80211_crypto_tkip_decrypt+0xc5/0x110 [mac80211]
  [<ffffffffa033102e>] ieee80211_rx_handlers+0x9bb/0x1fe1 [mac80211]

Fixes: 13303c0f ("iwlwifi: mvm: use helpers to get iwl_mvm_sta")
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>