net: macb: Flush correct cache portion when sending

The end address of the cache flush must be cache-line-aligned since
otherwise (at least on ARM926-EJS) the request is ignored. When the cache
is enabled this means that packets are not sent.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>