smsc75xx: Check for Wake-on-LAN modes

The driver does not check for Wake-on-LAN modes specified by an user,
but will conditionally set the device as wake-up enabled or not based on
that, which could be a very confusing user experience.

Fixes: 6c636503 ("smsc75xx: add wol magic packet support")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>