net: Access mapped physmem in net functions

Previously the net functions would access memory assuming physmem did
not need to be mapped.  In sandbox, that's not the case.

Now we map the physmem specified by the user in loadaddr to the buffer
that represents that space.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>