ath9k: Remove some #defined constants to decrease verbosity

The removed ATH9K_SLOT_TIME_X constants simply map the value in microseconds
to the same integer. These constants were not used consistently, so fix the
inconsistency issue by replacing all occurances with the integer equivalent.

Signed-off-by: Benjamin Berg <benjamin.berg@open-mesh.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>