ath9k: rename ath_rc_ratefind_ht() to ath_rc_get_highest_rix()

The purpose is to find the highest rate we can use.

Cc: Derek Smithies <derek@indranet.co.nz>
Cc: Chittajit Mitra <Chittajit.Mitra@Atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>