bnx2x: Use appropriate define for vlan credit

Although it has same value as MAX_MAC_CREDIT_E2,
use MAX_VLAN_CREDIT_E2 appropriately.

Signed-off-by: Manish Chopra <manishc@marvell.com>
Signed-off-by: Ariel Elior <aelior@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>