Bluetooth: Use macro for L2CAP hint mask on receiving config request

Using the L2CAP_CONF_HINT macro is easier to understand than using a
hardcoded 0x80 value.

Signed-off-by: Gustavo F. Padovan <gustavo@las.ic.unicamp.br>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>