mac80211: agg-tx: refactor sending addba

We move the actual arming the timer and sending ADDBA to a function
for the use in different places calling the same logic.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200326150855.58a337eb90a1.I75934e6464535fbf43969acc796bc886291e79a5@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>