drivers/net: decouple ISA and ISA_DMA_API

The two options are separate, and some platforms (e.g. arm pxa)
have ISA slots but no ISA dma controller, so they cannot build
drivers using the DMA API functions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>