ath5k: Fix functions for getting/setting slot time

Functions ath5k_hw_get_slot_time and ath5k_hw_set_slot_time were
converting microseconds to clocks only for AR5210, although it's needed
for all supported devices. The conversion was moved outside the
hardware-specific branches.

The original code also limited minimum slot time to 9, while turbo modes
use 6, this was fixed too.

Signed-off-by: Lukas Turek <8an@praha12.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>