Fix Wake on LAN support in sis900

Fix two bugs in the WoL implementation of sis900. The first causes
hangs on some system on driver load, the second causes troubles
when disabling WoL support.  Both fixes are one liner and really
simple.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Daniele Venzano <venza@brownhat.org>