USB: composite: Release spinlock before WARN()

if WARN() is called under spinlock, it will lead crash due to
watchdog timeout. Hence fix it by releasing spinlock before calling
WARN(). And also check cdev->config is NULL or not before calling
WARN().

Change-Id: Iebd00932b94164e9f40aa6126a2352437b1141ed
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>