iwlwifi: mvm: provide helper to fetch the iwl_mvm_sta from sta_id

We somtimes need to fetch the iwl_mvm_sta structure from a
station index - provide a helper to do that.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

diff --cc drivers/net/wireless/iwlwifi/mvm/bt-coex.c
Simple merge

diff --cc drivers/net/wireless/iwlwifi/mvm/debugfs.c
Simple merge

diff --cc drivers/net/wireless/iwlwifi/mvm/mvm.h
Simple merge