ath9k: remove ath9k_hw_chan2wmode()

The only left users are for timing for ACK timeout, slotime and
CTS timeout. We currently use an array CLOCK_RATE to keep
these values per mode and since as only will use A and G
we can depend on the band to get the appropriate values.

We note that we should be using a different clock rate value
for CCK, we can do this in separate patch, currently this is
being disregarded and should only affect when we want to
change the default ACK/CTS timeout or slot time and stuck
with using using 802.11b.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>