net: w5100: add ability to support other bus interface

The w5100 driver currently only supports direct and indirect bus
interface mode which use MMIO space for accessing w5100 registers.

In order to support SPI interface mode which is supported by W5100 chip,
this makes the bus interface abstraction layer more generic so that
separated w5100-spi driver can use w5100 driver as core module.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>