usb: gadget: dummy_hcd: fix null-deref free req

_ep to ep is a pointer substraction so ep won't be zero unless _ep was
8. This was not intendent by the author, it was probably a typo while
checking for NULL of the argument.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>