usblp: Make use of URB_FREE_BUFFER

Employ the new API URB_FREE_BUFFER that we've got. There was talk of a combined
constructor for this case, but apparently it's not happening, so just set the
flag explicitly for now.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>