Bluetooth: When encryption is dropped, do not send RFCOMM packets

During a role change with pre-Bluetooth 2.1 devices, the remote side drops
the encryption of the RFCOMM connection. We allow a grace period for the
encryption to be re-established, before dropping the connection. During
this grace period, the RFCOMM_SEC_PENDING flag is set. Check this flag
before sending RFCOMM packets.

Signed-off-by: Jaikumar Ganesh <jaikumar@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>