usb: gadget: composite: fix NULL pointer when a non standard request is received

In case usb configuration is unknown (cdev->config == NULL), non standard
request should not be processed.
Remove also the cdev->config check below which will never happen.

This issue was seen using ums feature.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>