Bluetooth: Rename L2CAP_LE_DEFAULT_MTU

This patch renames L2CAP_LE_DEFAULT_MTU macro to L2CAP_LE_MIN_MTU
since it represents the minimum MTU value, not the default MTU
value for LE.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>