ath6kl: Report unique remain-on-channel cookie values

Even though only a single concurrent remain-on-channel operation is
supported, there may be two pending remain-on-channel events (one to
indicate end of a canceled operation and another to indicate start of a
new operation). User space won't be able to distinguish these events
unless unique cookies are used.

The previous behavior resulted in wpa_supplicant getting quite
confused about the driver's offchannel state in various sequences
and this made the P2P state machine behave incorrectly. Use of
more than a single remain-on-channel cookie value fixes this.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>