net: phy: mxl-gpy: Change gpy_update_interface() function return type

gpy_update_interface() is called from gpy_read_status() which does
return error codes. gpy_read_status() would benefit from returning
-EINVAL, etc.

Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>