ntb_netdev: Fix list_for_each_entry exit issue

If list_for_each_entry exits without finding the ntb_device, the dev
pointer will not be NULL.  Thus the check will never be true and the
code will not exit when it should.  Correct this by adding a bool to
determine when the device is found, otherwise exit in good fashion.

Signed-off-by: Jon Mason <jon.mason@intel.com>