staging: rtl8723au: issue_action_BSSCoexistPacket(): Use struct ieee80211_mgmt

This gets rid of the rtw_set_fixed_ie23a() usage here. Note that this
abuses ieee80211_mgmt.u.action.u.chan_switch for now, since the
correct defines are currently missing, and this one matches.

Need to fix up the definitions in ieee80211.h and afterwards fix it up
here.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>