cxgb4: Fix for write-combining stats configuration

The write-combining configuration register SGE_STAT_CFG_A needs to
be configured after FW initializes the adapter, else FW will reset
the configuration

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