drivers: musb_gadget: Save endpoint desc to usb_ep->desc

Fix fastboot flash bug.

If the downloading file size is equal to the partition size, "fastboot
flash" can't work, at least in sunxi platform, because used an
uninitalized point: ep->desc.

This patch also fixed 'data abort' bug in am335x platform.

Reproduce: fastboot flash loader1 spl/sunxi-spl.bin.

Signed-off-by: qianfan Zhao <qianfanguijin@163.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>