net: moxart: use correct accessors for DMA memory

The moxart ethernet driver confuses coherent DMA buffers with
MMIO registers.

moxart_ether.c: In function 'moxart_mac_setup_desc_ring':
moxart_ether.c:146:428: error: passing argument 1 of '__fswab32' makes integer from pointer without a cast [-Werror=int-conversion]
moxart_ether.c:74:39: warning: incorrect type in argument 3 (different address spaces)
moxart_ether.c:74:39:    expected void *cpu_addr
moxart_ether.c:74:39:    got void [noderef] <asn:2>*tx_desc_base

This leaves the basic logic alone and uses normal pointers for
the virtual address of the descriptor. As we cannot use readl/writel
to access them, we also introduce our own moxart_desc_read
moxart_desc_write helpers that perform the same endianess swap
as the original code, but without the address space conversion.

The barriers are made explicit here where needed: Even in the worst-case
scenario, we just have to use a rmb() after checking ownership so
we don't read any input data before we are sure it is value, and we
use wmb() before transferring ownership back to the device.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>