rtwn(4): clear 'basic' rate bit before calculating RTS/CTS rate.

Rate tables have this bit set to indicate minimal set of basic rates;
however, it overlappes with MCS bit, so rate2ridx() will treat them as
an 11n rate.

Due to the current rates setup the issue can be reproduced only
in 5GHz band with 11n / protection enabled.

Tested with RTL8821AU, HOSTAP mode.

MFC after: 5 days