fix bug where a register which was only meant to be read in 578xx/57712
devices causes a bogus error message to be logged when read from other
devices.

Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>