net: w5100: don't build spi driver without w5100

The w5100-spi driver front-end only makes sense when the w5100
core driver is enabled, not for a configuration that only has w5300:

drivers/net/built-in.o: In function `w5100_spi_remove':
drivers/net/ethernet/wiznet/w5100-spi.c:277: undefined reference to `w5100_remove'
drivers/net/built-in.o: In function `w5100_spi_probe':
drivers/net/ethernet/wiznet/w5100-spi.c:272: undefined reference to `w5100_probe'
drivers/net/built-in.o: In function `w5200_spi_init':
drivers/net/ethernet/wiznet/w5100-spi.c:125: undefined reference to `w5100_ops_priv'
drivers/net/built-in.o: In function `w5200_spi_readbulk':
drivers/net/ethernet/wiznet/w5100-spi.c:125: undefined reference to `w5100_ops_priv'
drivers/net/built-in.o: In function `w5200_spi_writebulk':
drivers/net/ethernet/wiznet/w5100-spi.c:125: undefined reference to `w5100_ops_priv'
drivers/net/built-in.o:(.data+0x3ed1c): undefined reference to `w5100_pm_ops'

This adds an appropriate Kconfig dependency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 630cf097 ("net: w5100: support SPI interface mode")
Signed-off-by: David S. Miller <davem@davemloft.net>