staging: rtl8723au: Move rtw_do_join23a() prototype to rtw_ioctl_set.h

rtw_do_join23a() is declared in rtw_ioctl_set.c so move the prototype
to it's correct place, and remove duplicate protoype from rtw_mlme.c

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