net: systemport: fix DMA allocation/freeing sizes

We should not be allocating a single byte of DMA coherent memory, but
instead a full-sized struct dma_desc (8 bytes).

Fixes: 80105befdb4b ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>