bnx2: Add bnx2_shutdown_chip().

This logic is used in bnx2_close() and bnx2_suspend() and
so should be separated out into a separate function.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>