[ath_rate_sample] Fix logic for determining whether to bump up an MCS rate.

• Fix formatting, cause reasons;
• Put back the "and the chosen rate is within 90% of the current rate" logic;
• Ensure the best rate and the current rate aren't the same; this ...
• ... fixes the packets_since_switch[] tracking to actually conut how many frames since the rate switched, so now I know how stable stuff is; and
• Ensure that MCS can go up to a higher MCS at this or any other spatial stream. My previous quick hack attempt was doing > rather than >= so you had to go to both a higher root MCS rate (0..7) and spatial stream. Eg, you couldn't go from MCS0 (1ss) to MCS8 (2ss) this way.

The best rate and switching rate logic still have a bunch more work to do
because they're still quite touchy when it comes to average tx time but at least
now it's choosing higher rates correctly when it wants to try a higher rate.

Tested:

• AR9380, STA mode