usb: udc_virtual: Adapt to updated API

Commit 00adb2a5 ("drivers: udc: remove no more required pending
state flag") removed state parameter from udc_buf_peek and udc_buf_get.
Adapt udc virtual driver so the code compiles.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>