bnx2x: Replace doorbell barrier() with wmb()

barrier() doesn't guarantee memory writes to be observed by the hardware on
all architectures. barrier() only tells compiler not to move this code
with respect to other read/writes.

If memory write needs to be observed by the HW, wmb() is the right choice.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>