ath6kl: Allow enabling of P2P support

For now, use a module parameter (ath6kl_p2p) to allow P2P support
to be enabled. This is needed since there is no mechanism for
enabling the P2P mode more dynamically for a single netdev.

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