ath9k: remove pointless wrapper ath_rc_rate_getidx()

This is just calling another helper, so just use the other
helper directly. This should make it clear that when do not
find the next rate we stick to the current one.

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>