dsa: mv88e6xxx: Kill the REG_READ and REG_WRITE macros

These macros hide a ds variable and a return statement on error, which
can lead to locking issues. Kill them off.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>