[BNX2]: Fix link interrupt problem.

bnx2_has_work()'s logic is flawed and can cause the driver to miss
a link event.  The fix is to compare the status block's attn_bits
and attn_bits_ack to determine if there is a link event.

Update version to 1.5.6.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>