AR8216: improve mmd register access

Combine all bus operations for one MMD access in one function.
Protecting all these bus operations with one lock also helps
to avoid potential issues due to bus operations intercepting
the register and data write.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48914 3c298f89-4303-0410-b956-a3cf2f4a3e73