ath5k: initialize common->clockrate

Fixes a division by zero when setting distance before activating the
device for the first time.

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