be2net: Warn users of possible broken functionality on BE2 cards with very old FW versions with latest driver

On very old FW versions < 4.0, the mailbox command to set interrupts
on the card succeeds even though it is not supported and should have
failed, leading to a scenario where interrupts do not work.
Hence warn users to upgrade to a suitable FW version to avoid seeing
broken functionality.
Signed-off-by: Somnath Kotur <somnath.kotur@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>