ath9k: FFT magnitude check: don't consider lower 3 data bits

There were a lot of Magnitude Mismatch while getting FFT samples on my
hardware (Atheros AR9462. I've compared the reported magnitude with
the data in the FFT bin, and the FFT bin was less accurate:

[ 5395.193030] ath: phy0: FFT HT20 frame: max mag 0x89,max_mag_idx 28,
,magnitude 0x89 max_exp 0, data[28] = 0x88
[ 5395.194525] ath: phy0: FFT HT20 frame: max mag 0x89,max_mag_idx 28,
,magnitude 0x89 max_exp 0, data[28] = 0x88
[ 5395.196012] ath: phy0: FFT HT20 frame: max mag 0x88,max_mag_idx 28,
,magnitude 0x88 max_exp 0, data[28] = 0x88
[ 5395.197509] ath: phy0: FFT HT20 frame: max mag 0x6C,max_mag_idx 28,
,magnitude 0x6C max_exp 0, data[28] = 0x68
[ 5395.199015] ath: phy0: FFT HT20 frame: max mag 0x78,max_mag_idx 28,
,magnitude 0x78 max_exp 0, data[28] = 0x78
[ 5395.200497] ath: phy0: FFT HT20 frame: max mag 0xA1,max_mag_idx 28,
,magnitude 0xA1 max_exp 0, data[28] = 0xA0
[ 5395.202011] ath: phy0: FFT HT20 frame: max mag 0x91,max_mag_idx 28,
,magnitude 0x91 max_exp 0, data[28] = 0x90
[ 5395.203482] ath: phy0: FFT HT20 frame: max mag 0x89,max_mag_idx 28,
,magnitude 0x89 max_exp 0, data[28] = 0x88
[ 5395.204999] ath: phy0: FFT HT20 frame: max mag 0x27,max_mag_idx 4,
,magnitude 0x27 max_exp 0, data[4] = 0x20
[ 5395.206461] ath: phy0: FFT HT20 frame: max mag 0x41,max_mag_idx 28,
,magnitude 0x41 max_exp 0, data[28] = 0x40
[ 5395.207977] ath: phy0: FFT HT20 frame: max mag 0x51,max_mag_idx 28,
,magnitude 0x51 max_exp 0, data[28] = 0x50
[ 5395.209454] ath: phy0: FFT HT20 frame: max mag 0x53,max_mag_idx 28,
,magnitude 0x53 max_exp 0, data[28] = 0x50
[ 5395.210940] ath: phy0: FFT HT20 frame: max mag 0x40,max_mag_idx 28,
,magnitude 0x40 max_exp 0, data[28] = 0x40
[ 5395.212441] ath: phy0: FFT HT20 frame: max mag 0x59,max_mag_idx 28,
,magnitude 0x59 max_exp 0, data[28] = 0x58
[ 5395.213932] ath: phy0: FFT HT20 frame: max mag 0x53,max_mag_idx 28,
,magnitude 0x53 max_exp 0, data[28] = 0x50
[ 5395.215428] ath: phy0: FFT HT20 frame: max mag 0x7D,max_mag_idx 28,
,magnitude 0x7D max_exp 0, data[28] = 0x78
[ 5395.216910] ath: phy0: FFT HT20 frame: max mag 0x8C,max_mag_idx 28,
,magnitude 0x8C max_exp 0, data[28] = 0x88
[ 5395.218413] ath: phy0: FFT HT20 frame: max mag 0x7B,max_mag_idx 28,
,magnitude 0x7B max_exp 0, data[28] = 0x78
[ 5395.219900] ath: phy0: FFT HT20 frame: max mag 0x43,max_mag_idx 28,
,magnitude 0x43 max_exp 0, data[28] = 0x40

It seems like the lower 3 bits on my hardware are always zeroed, but the
magnitude matches otherwise. Therefore, let's not make the magnitude
check so strict so we can get those samples released to userspace.

Cc: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>