Bluetooth: Add channel policy to getsockopt/setsockopt

Each channel has a policy to require BR/EDR (the default),
prefer BR/EDR, or prefer AMP.

Check for valid policy value and L2CAP mode.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Acked-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>