LinuxKPI: 802.11: factor out dynamic_rx_chains computation

For the moment we set dynamic_rx_chains to the rx_nss value which
should be correct given we count from 1 there too and we call
lkpi_get_max_rx_chains() after we synced ht/vht initially already.

Long-term some of these counting bits should be further factored
out into their own functions and used both here and for the sync
code.

This will likely want to change as well once more dyamic
synncronization takes place.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days