net: designware: fix descriptor layout and warnings on 64-bit archs

All members of the DMA descriptor must be 32-bit, even on 64-bit
architectures: change the type to u32 to ensure this. Also, fix
other warnings.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Use phys_addr_t not unsigned long long to test that we're within
DMA'able memory]
Signed-off-by: Tom Rini <trini@konsulko.com>