Change spi member of struct mcp23s08 to be a ops-specific opaque data
pointer, and move spi specific knowledge out of mcp23s08_probe_one().

No functional change, but is needed to add i2c support.
Signed-off-by: NPeter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>