ath9k_hw: initialize ah->slottime

(u32) -1 is not particularly useful as a slottime default, so even though
the ath9k_hw default should never get used, it's better to pick something
sane here.

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