ath9k: fix invalid max frame length

According to 802.11n-2012 standard in paragraph PPDU Fromat(20.3.2) HT-mixed
format  Hearder PPDU contains : L_STF, L_LTF, L_SIG, HT_SIG, HT_STF, HT_LTF
they are symbols in the preamble, there are in time unit(us) that's for why
it can't be computed in bytes

Signed-off-by: Sylvain ROGER RIEUNIER <sylvain.roger.rieunier@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -1040,11 +1040,11 @@ static int ath_max_framelen(int usec, int mcs, bool ht40, bool sgi)
 	int symbols, bits;
 	int bytes = 0;
 
+	usec -= L_STF + L_LTF + L_SIG + HT_SIG + HT_STF + HT_LTF(streams);
 	symbols = sgi ? TIME_SYMBOLS_HALFGI(usec) : TIME_SYMBOLS(usec);
 	bits = symbols * bits_per_symbol[mcs % 8][ht40] * streams;
 	bits -= OFDM_PLCP_BITS;
 	bytes = bits / 8;
-	bytes -= L_STF + L_LTF + L_SIG + HT_SIG + HT_STF + HT_LTF(streams);
 	if (bytes > 65532)
 		bytes = 65532;