cxgb4: Query firmware for T5 ULPTX MEMWRITE DSGL capabilities

Query firmware to see whether we're allowed to use T5 ULPTX MEMWRITE DSGL
capabilities.  Also pass that information to Upper Layer Drivers via the
new (struct cxgb4_lld_info).ulptx_memwrite_dsgl boolean.

Based on original work by Casey Leedom <leedom@chelsio.com>

Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>