bnx2: don't request firmware when there's no userspace.

The firmware is cached during the first successful call to open() and
released once the network device is unregistered. The driver uses the
cached firmware between open() and unregister_netdev().

It's similar to 953a12cc2889d1be92e80a2d0bab5ffef4942300 but the
firmware is mandatory.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>