usb/isp1760: Remove redundant variables and defines

Removes the redundant hw_next list pointer from struct isp1760_qtd, removes some
unused #defines, removes redundant "urb" member from struct inter_packet_info.

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>