mac80211: refactor sta_info_insert_rcu to 3 main stages

Divided the sta_info_insert_rcu function to 3 mini-functions:
sta_info_insert_check - the initial checks done when inserting
a new station
sta_info_insert_ibss - the function that handles the station
addition for IBSS interfaces
sta_info_insert_non_ibss - the function that handles the station
addition in other cases

The outer API was not changed.
The refactoring was done for better usage of the different
stages in the station addition in new scenarios added
in the next commit.

Signed-off-by: Guy Eilam <guy@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>