net80211(4): do not setup roaming parameters for unsupported modes.

ifconfig(8) prints per-mode parameters if they are non-zero; since
we have 13 possible modes with 3...5 typically supported this change
should greatly reduce amount of information for 'ifconfig <wlan> list roam'
command.

While here ensure that sta_roam_check() will not use roaming parameters
for unsupported modes (it should not).

This change effectively reverts r188776.

MFC after: 2 weeks