net: dsa: mv88e6060: Replace REG_READ macro

The REG_READ macro contains a return statement, making it not very
safe. Remove it by inlining the code.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>