spi/intel: move driver_api assignment later

This moves the assignment of driver_api to just before
the init function returns. This is in preparation to
make device_get_binding() return NULL if driver
initialization fails.

Change-Id: I69590c463b84877d250c63d4460b7e254b79c8b3
Signed-off-by: Daniel Leung <daniel.leung@intel.com>