[Bluetooth] Allow disabling of credit based flow control

This patch adds the module parameter disable_cfc which can be used to
disable the credit based flow control. The credit based flow control
was introduced with the Bluetooth 1.1 specification and devices can
negotiate its support, but for testing purpose it is helpful to allow
disabling of it.

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