linux80211: fix default deflink.rx_nss

Native Linux implementation sets this as a maximum between 1 and
ht/vht/eht rx SS'es, FreeBSD does the same, but uses 0 as a minimum,
which leads setting it to 0 if we're not in ht/vht case.

This 0 was breaking rtw89 driver, when it was trying to determine
SS number by subtracting 1 from rx_nss and passing the value to the
hardware.

After this patch rtw89 association and simple ping work reliably,
but more work is needed to make the driver robust with heavy traffic
(iperf3) and being long idle.

Reviewed by: bz
Approved by: bz
Sponsored by: Future Crew LLC
Differential Revision: https://reviews.freebsd.org/D46528