net: dsa: mt7530: introduce mutex helpers

As the MDIO bus lock only needs to be involved if actually operating
on an MDIO-connected switch we will need to skip locking for built-in
switches which are accessed via MMIO.
Create helper functions which simplify that upcoming change.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>