usb/isp1760: Simpler queue head list code.

Small code refactoring to ease the real fix in patch #2.

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>