Bluetooth: Only enable L2CAP FCS for ERTM or streaming

This fixes a bug which caused the FCS setting to show L2CAP_FCS_CRC16
with L2CAP modes other than ERTM or streaming.  At present, this only
affects the FCS value shown with getsockopt() for basic mode.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>