ath9k: Fix offchannel duration calculation

Currently, different units are used for handling
sc->offchannel.duration. In scan mode, it contains jiffies and in RoC
mode, milliseconds is used. This causes confusion since in
ath_chanctx_switch(), TU_TO_USEC is used to determine the offchannel
duration, resulting in incorrect values. Fix this by using jiffies in
both modes.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>