ath9k: remove ATH9K_MODE_11B

This saves us 2733 bytes.

   text    data     bss     dec     hex filename
 252265    3628    1584  257477   3edc5 ath9k-has-b-rate.ko
 249905    3628    1584  255117   3e48d ath9k.ko

Cc: Derek Smithies <derek@indranet.co.nz>
Cc: Chittajit Mitra <Chittajit.Mitra@Atheros.com>
Siged-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>

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