avoid using MCS9 in iwm(4) Tx rate selection if 40 MHz is disabled

avoid using MCS9 in iwm(4) Tx rate selection if 40 MHz is disabled

MCS9 is not valid on 20HMz so we must not use it in 20MHz-only mode.
Fixes firmware panic 0x00002078, which indicates the driver sets a rate
with a bad number of antennas, bad number of Tx streams, or bad bandwidth.

Observed by dlg@ with the miktrotik AP for which iwm(4) firmware requires
our 20MHz-only workaround to interoperate in 11ac mode.

ok dlg@