cxgb4: Handle clip return values

Add a warn message when clip table overflows. If clip table isn't
allocated, return from cxgb4_clip_release() to avoid panic.
Disable offload if clip isn't enabled in the hardware.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>