ath_rate_sample: correct the "best rate" calculation

This should be a *9 rather than a *10 so higher stream MCS rates
(eg comparing MCS0 and MCS8) that have slightly longer average transmit
times (but better burst transmit times) get considered.

This mirrors what the later code does when considering if a rate
change is needed.

Locally tested:

• AR9280, AP mode
• AR9380, AP mode

Differential Revision: https://reviews.freebsd.org/D47988
Reviewed by: imp