tg3: Introduce separate functions to allocate/free RX/TX rings.

This is preparation work to allow the number of RX and TX rings to be
configured separately.

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