ath9k: cleanup slot time and ack/cts timeout handling

Previously ath9k left the initialization of slot timing and ACK/CTS
timeout to the mode specific initvals. This does not handle short vs
long slot in 2.4 GHz and uses a rather strange value for the 2.4 GHz
ACK timeout (64 usec).
This patch uses the proper ath9k_hw functions for setting slot time and
timeouts and also implements the switch between short and long slot
time in 2.4 GHz

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>