net: wan: slic_ds26522: add SPI device ID table to fix module autoload

If the driver is built as a module, module alias information isn't filled
so the module won't be autoloaded. Add a SPI device ID table and use the
MODULE_DEVICE_TABLE() macro so the information is exported in the module.

Before this patch:

$ modinfo drivers/net/wan/slic_ds26522.ko | grep alias
$

After this patch:

$ modinfo drivers/net/wan/slic_ds26522.ko | grep alias
alias:          spi:ds26522

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>