lan78xx: Add missing return code checks

There are many places in the driver where the return code from a
function call is captured but without a subsequent test of the
return code and appropriate action taken.

This patch adds the missing return code tests and action. In most
cases the action is an early exit from the calling function.

The function lan78xx_set_suspend() was also updated to make it
consistent with lan78xx_suspend().

Signed-off-by: John Efstathiades <john.efstathiades@pebblebay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>