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>