spi: Clean up probe and remove functions

While backporting 33cf00e5 ("spi: attach/detach SPI device to the ACPI
power domain"), I noticed that the code changes were suboptimal:

* Why use &spi->dev when we have dev at hand?

* After fixing the above, spi is used only once, so we don't really
  need a local variable for it.

This results in the following clean-up.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>