ath5k: Add new field on ath5k_hw to track bandwidth modes

 * Prepare for half/quarter/turbo support, introduce a new
 ah_bwmode parameter and get rid of ah_turbo. Bwmode stands
 for "bandwidth mode" and can have 4 values, default (20MHz),
 turbo (40MHz), half rate (10MHz), and quarter rate (5MHz).

 Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>