ath6kl: fix busreqs so they can be reused when sg is cleaned up

To reuse the busreqs in case of hardware restart, they must be
properly reinitialized.  If the scat_req pointer isn't reset to
0, __ath6kl_sdio_write_async() will assume there is sg work to be
done (causing a kernel OOPS).

Signed-off-by: James Minor <james.minor@ni.com>
Reviewed-by: Steve deRosier <steve.derosier@lairdtech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>