mac80211: Fix power save in change interface

we found that power save is not getting enabled when we do
change interface in this order STA->IBSS->STA. this is
because ieee80211_setup_sdata clears type-dependent union

Reported-by: Leela Kella <leela@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>