Fix kern/196290 - don't announce 11n HTINFO rates if the channel is
configured as 11b.

This came up when debugging other issues surrounding scanning and
channel modes.

What's going on:

• The VAP comes up as an 11b VAP, but on an 11n capable NIC;
• .. it announces HTINFO and MCS rates;
• The AP thinks it's an 11n capable device and transmits 11n frames to the STA;
• But the STA is in 11b mode, and thus doesn't receive/ACK the frames.

It didn't happen for the ath(4) devices as the AR5416/AR9300 HALs
unconditionally enable MCS frame reception, even if the channel
mode is not 11n. But the Intel NICs are configured in 11b/11a/11g
modes when doing those, even if 11n is enabled and available.

So, don't announce 11n capabilities if the VAP isn't on an 11n
channel when sending management assocation request / reassociation
request frames.

TODO:

• Lots more testing - 11n should be "upgraded" after association, and I just want to make sure I haven't broken 11n upgrade. I shouldn't have - this is only happening for /sending/ association requests, which APs aren't doing.

Tested:

• ath(4) APs (AR9331, AR7161+AR9280, AR934x)
• AR5416, STA mode
• Intel 5100, STA mode

PR: kern/196290