chelsio: cxgb4: fix sparse warning

this patch fixes following sparse warning:

cxgb4_dcb.c:25:6: warning: symbol 'dcb_ver_array' was not declared. Should it be static?

Alongside making it const.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>