usb: dwc3: ep0: return early on NULL requests
usb: dwc3: ep0: return early on NULL requests

if our list of requests is empty, return early.

There's really nothing to be done in case our
request list is empty anyway because the only
situation where we our list is empty, is when
we're transferring ZLPs.

Signed-off-by: Felipe Balbi <94dddeeef08b001e003cce128ddc162a4e2c6cd2@ti.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs