cxgb4: properly initialize variables

memset variables to 0 to fix sparse warnings:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_cudbg.c:409:42: sparse: Using
plain integer as NULL pointer

drivers/net/ethernet/chelsio/cxgb4/cudbg_zlib.c:43:47: sparse: Using
plain integer as NULL pointer

Fixes: ad75b7d3 ("cxgb4: implement ethtool dump data operations")
Fixes: 91c1953d

 ("cxgb4: use zlib deflate to compress firmware dump")
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>