drivers/net/rtl8139.c: Fix cache coherency issues

Current driver is meant for cache coherent systems. This patch adds
flush_cache() routines to support cache non-coherent systems.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Cc: Masami Komiya <mkomiya@sonare.it>
Cc: Lucas Jin <lucasjin@gmail.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>