bnx2: Put rx ring variables in a separate struct.

In preparation for multi-ring support, rx ring variables are now put
in a separate bnx2_rx_ring_info struct.  With MSI-X, we can support
multiple rx rings.

The functions to allocate/free rx memory and to initialize rx 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>