ath9k: rename ath_rc_get_nextlowervalid_txrate()

What this does is get us our next lower rate so call it that,
ath_rc_get_lower_rix().

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>