bnx2x: Load firmware in open() instead of probe()

Loading firmware when actually bringing eth device up.
This also will allow driver to be insmoded when filesystem with
firmware files is not available yet.

Suggested by Stephen Hemminger <shemminger@vyatta.com>

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>