dsa: fix compile bug on s390

git commit 45cec1ba
"dsa: Need to select PHYLIB." causes this build bug on s390:

drivers/built-in.o: In function `phy_stop_interrupts':
/home/heicarst/linux-2.6/drivers/net/phy/phy.c:631: undefined reference to `free_irq'
/home/heicarst/linux-2.6/drivers/net/phy/phy.c:646: undefined reference to `enable_irq'
drivers/built-in.o: In function `phy_start_interrupts':
/home/heicarst/linux-2.6/drivers/net/phy/phy.c:601: undefined reference to `request_irq'
drivers/built-in.o: In function `phy_interrupt':
/home/heicarst/linux-2.6/drivers/net/phy/phy.c:528: undefined reference to `disable_irq_nosync'
drivers/built-in.o: In function `phy_change':
/home/heicarst/linux-2.6/drivers/net/phy/phy.c:674: undefined reference to `enable_irq'
/home/heicarst/linux-2.6/drivers/net/phy/phy.c:692: undefined reference to `disable_irq'

PHYLIB has alread a depend on !S390, however select PHYLIB at DSA overrides
that unfortunately. So add a depend on !S390 to DSA as well.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>