rtw88: add ieee80211_ops::change_interface

Sometimes mac80211 will just ask driver to change the interface
instead of to remove and then add a new one. And for driver, to
change the interface is just simply to remove and add a new one
but change the type of it.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200717064937.27966-6-yhchuang@realtek.com