ath6kl: Return a proper error code when not in connected state

Error code ENOTCONN is more suitable than EINVAL to report
when the driver is not in connected state in ath6kl_wow_suspend().

I found this during code review.

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>