nfp: fix NFP_NET_MAX_DSCP definition error

The patch corrects the NFP_NET_MAX_DSCP definition in the main.h file.

The incorrect definition result DSCP bits not being mapped properly when
DCB is set. When NFP_NET_MAX_DSCP was defined as 4, the next 60 DSCP
bits failed to be set.

Fixes: 9b7fe804

 ("nfp: add DCB IEEE support")
Cc: stable@vger.kernel.org
Signed-off-by: Huayu Chen <huayu.chen@corigine.com>
Acked-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>