Bluetooth: Move set of P-bit to l2cap_send_sframe()

Abstract the send of of P-bit and avoids code duplication like we
did with the setting of F-bit.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>