sis900_mii_probe() must be __devinit

This patch fixes the following section mismatch:

<--  snip  -->

...
  MODPOST vmlinux
WARNING: drivers/built-in.o(.text+0x298170): Section mismatch: reference to .init.text:sis900_mii_probe (between 'sis900_probe' and 'sis900_default_phy')
...

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>