libpayload: Don't try to free individual xhci device slots

`di` points to a single item in xhci->dev[], which is malloc'd
collectively. Trying to free() leads to pain.

Change-Id: Ibd99eda905d43cbf2d2c111dfd0186ed6b119329
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Found-by: Coverity Scan
Reviewed-on: http://review.coreboot.org/8515
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/drivers/usb/xhci_devconf.c

@@ -218,7 +218,6 @@ _free_return:
 	free(tr);
 	if (di)
 		free(di->ctx.raw);
-	free((void *)di);
 _free_ic_return:
 	if (ic)
 		free(ic->raw);