ath5k: fix reference clock usec duration setting restore

enabling the sleep clock alters the AR5K_USEC_32 field, but disabling
it didn't restore it.

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