bnx2: Put tx ring variables in a separate struct.

In preparation for multi-ring support, tx ring variables are now put
in a separate bnx2_tx_ring_info struct.  Multi tx ring will not be
enabled until it is fully supported by the stack.  Only 1 tx ring
will be used at the moment.

The functions to allocate/free tx memory and to initialize tx rings
are now modified to handle multiple rings.

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