Bluetooth: sco: new getsockopt options BT_SNDMTU/BT_RCVMTU

This patch defines new getsockopt options BT_SNDMTU/BT_RCVMTU
for SCO socket to be compatible with other bluetooth sockets.
These new options return the same value as option SCO_OPTIONS
which is already present on existing kernels.

Signed-off-by: Joseph Hwang <josephsih@chromium.org>
Reviewed-by: Alain Michaud <alainm@chromium.org>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Reviewed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>