[Bluetooth] Reduce L2CAP MTU for RFCOMM connections

This patch reduces the default L2CAP MTU for all RFCOMM connections
from 1024 to 1013 to improve the interoperability with some broken
RFCOMM implementations. To make this more flexible the L2CAP MTU
becomes also a module parameter and so it can changed at runtime.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>