mv643xx_eth: move rx_return_buff() into its only caller

rx_return_buff() is also a remnant of the HAL layering that the
original mv643xx_eth driver used.  Moving it into its caller kills
the last reference to FUNC_RET_STATUS/pkt_info.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>