staging: rtl8188eu: remove "rtw_ieee80211_back_actioncode"

This enum is the same as "ieee80211_back_actioncode" in
"include/linux/ieee80211.h".

Update the code accordingly and remove useless comment.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/6b6d8d81e7b2115e9206d2960ce09e1b2e8b6381.1624132543.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>