bnx2x: Prevent VF race

The mail box containing the Vf-Pf messages is susceptible
to a race - it's possible for 2 flows to try and write commands,
causing one to override the other's message.
Use a mutex to synchronize the access, preventing said race.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>