rtwn: allow firmware rate control to be enabled for rtl8192cu

Although there's no RTS/CTS rate control available yet, the support
is enough to enable firmware rate control for experimenting.

This won't be enabled by default - users will need to set a tunable
before loading the driver (eg kenv dev.rtwn.0.ratectl=2) but it is
enough for experimentation, feedback and continued work.

Locally tested:

* RTL8192CU, STA mode

Differential Revision:	https://reviews.freebsd.org/D48143
Reviewed by:	bz, emaste

sys/dev/rtwn/rtl8192c/usb/r92cu_init.c

@@ -373,11 +373,7 @@ r92cu_post_init(struct rtwn_softc *sc)
 	if (sc->sc_flags & RTWN_FW_LOADED) {
 		struct r92c_softc *rs = sc->sc_priv;
 
-		if (sc->sc_ratectl_sysctl == RTWN_RATECTL_FW) {
-			/* XXX firmware RA does not work yet */
-			sc->sc_ratectl = RTWN_RATECTL_NET80211;
-		} else
-			sc->sc_ratectl = sc->sc_ratectl_sysctl;
+		sc->sc_ratectl = sc->sc_ratectl_sysctl;
 
 		/* Start C2H event handling. */
 		callout_reset(&rs->rs_c2h_report, rs->rs_c2h_timeout,