Bluetooth: Fix initial RFCOMM DLC security level

Due to commit 63ce0900 connections initiated through TTYs created with
"rfcomm bind ..." would have security level BT_SECURITY_SDP instead of
BT_SECURITY_LOW. This would cause instant connection failure between any
two SSP capable devices due to the L2CAP connect request to RFCOMM being
sent before authentication has been performed. This patch fixes the
regression by always initializing the DLC security level to
BT_SECURITY_LOW.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Acked-by: Luiz Augusto von Dentz <luiz.dentz-von@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>