usb: gadger: f_fs: Do not copy past descriptor end.

Endpoint descriptors come in 2 sizes, struct usb_endpoint_descriptor being
the largest. Use bLength to stop on endpoint descriptor boundary, and not
2 bytes too far.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>