diag: Enable diagfwd via USB even when MHI is available

Some devices need diag via USB even when MHI is used
for rmnet and qmi tethering. This is because of lack
of support for diagfwd for mhi_dev in Linux.

Change-Id: Ie0ec19ee109d49aa409d1df3ad5ca24b75de7102
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>