bnx2x: fix pf2vf bulletin DMA mapping leak

When freeing VF's DMA mappings, an already NULLed pointer was checked
again due to an apparent copy&paste error. Consequently, the pf2vf
bulletin DMA mapping was not freed.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Acked-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>