ath9k: Maintain rate table choice after association

A scan run after association would change sc_curmode which is
used to get the current rate table. This patch fixes it
by removing sc_curmode and setting the rate table in usage in cur_rate_table
on association.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>