cxgb4: zero the HMA memory

firmware expects HMA memory to be zeroed, use __GFP_ZERO
for HMA memory allocation.

Fixes: 8b4e6b3c ("cxgb4: Add HMA support")
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>