s390: drop CONFIG_VIRT_TO_BUS

VIRT_TO_BUS is only used for legacy device PCI and ISA drivers using
virt_to_bus() instead of the streaming DMA mapping API, and the
remaining drivers generally don't work on 64-bit architectures.

Two of these drivers also cause a build warning on s390, so instead
of trying to fix that, let's just disable the option as we do on
most architectures now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>