cxgb4/iw_cxgb4: Move common defines to cxgb4

This define is used by cxgb4i and iw_cxgb4, moving to avoid code duplication

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>