net: mvmdio: simplify the smi read and write error paths

Cosmetic patch simplifying the smi read and write error paths. It also
align their error paths with the ones of the xsmi functions.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>