net: systemport: fix dma_unmap_single() len

dma_unmap_single() was called with dma_unmap_len(cb, dma_len),
unfortunately we failed to assign this length field in
bcm_sysport_rx_refill() or bcm_sysport_alloc_rx_bufs() using
dma_unmap_len_set().

This causes packet contents corruption because are we not invoking the
cache invalidation routines with the proper length.  Fix this by using
the full RX buffer size (RX_BUF_LENGTH) because the mappings for the RX
bufers are created with that size.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>