net80211(4): fix rate check when 'roaming' ifconfig(8) option is set to 'auto'

Do not try to clear 'basic rate' bit from roamRate; it cannot be here and,
actually, this operation clears 'MCS rate' bit instead, breaking comparison
for 11n / 11ac modes.

Tested with RTL8188CUS, HOSTAP mode + RTL8821AU, STA mode.

MFC after: 3 days