usb: xhci: dbc: Simplify error handling in 'xhci_dbc_alloc_requests()'

If the 'kmalloc()' fails, we need to undo the previous
'dbc_alloc_request()' call.

Because of the more similar function name, it is more logical to use
'dbc_free_request()' instead of 'xhci_dbc_free_req()'.

Both are equivalent here because:
 static void xhci_dbc_free_req(struct dbc_ep *dep, struct dbc_request *req)
 {
 	kfree(req->buf);
 	dbc_free_request(dep, req);
 }
and 'req->buf' is known to be NULL at this point

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/1567172356-12915-2-git-send-email-mathias.nyman@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>