imx_usb: print USB information only if DEBUG is set
imx_usb: print USB information only if DEBUG is set

Lots of libusb_interrupt_transfer messages where shown which are
actually debug message, so print them conditionally only if DEBUG
is defined.

Adellica/imx_usb_loader,boundarydevices/imx_usb_loader,lucaceresoli/imx_usb_loader,pacificIT/imx_usb_loader,vstehle/imx_usb_loader,bwalle/imx_usb_loader