USB: Add URB_FREE_BUFFER flag and the logic behind it

USB: Add URB_FREE_BUFFER flag for freeing the transfer buffer

In some cases it is not needed that the driver keeps track of the
transfer buffer of an URB. It can be simply freed along with the
URB itself when the reference count goes down to zero. The new
flag URB_FREE_BUFFER enables this behavior.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>