usb: configfs: Add null check for function config in android setup

When composition switch fails and composition is fallen to previous
composition, there is a chance that function config is null in
android setup which leads to null pointer dereference in function
driver. To avoid that add null check for function config in android
setup.

Change-Id: I922d279fcab6d2b1e945f1ba718cd40aafa3c2cb
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>