cxgb4: fix overflow in collecting IBQ and OBQ dump

Destination buffer already has offset added.  So, don't add offset
again.

Fetch actual size of configured OBQ from hardware, instead of using
hardcoded value.

Fixes: 7c075ce2

 ("cxgb4: collect IBQ and OBQ dumps")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>