bnx2: Fix bug in bnx2_free_tx_skbs().

In rare cases, bnx2x_free_tx_skbs() can unmap the wrong DMA address
when it gets to the last entry of the tx ring.  We were not using
the proper macro to skip the last entry when advancing the tx index.

Reported-by: Zongyun Lai <zlai@vmware.com>
Reviewed-by: Jeffrey Huang <huangjw@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>