tg3: rename rx_[std|jmb]_ptr

A later patch is going to add consumer indicies for the producer rings.
To keep things readable, this patch renames rx_[std|jmb]_ptr to
rx_[std|jmb]_prod_idx.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>