mcp23s08: isolate spi specific parts

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: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>