net: mvneta: rely on MVNETA_MAX_RX_BUF_SIZE for pkt split in mvneta_swbm_rx_frame()

In order to easily change the rx buffer size, rely on
MVNETA_MAX_RX_BUF_SIZE instead of PAGE_SIZE in mvneta_swbm_rx_frame
routine for rx buffer split. Currently this is not an issue since we set
MVNETA_MAX_RX_BUF_SIZE to PAGE_SIZE - MVNETA_SKB_PAD but it is a good to
have to configure a different rx buffer size.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>