spi: update modalias_show after of_device_uevent_modalias support

Commit 3ce6c9e2 ("spi: add of_device_uevent_modalias support") is
incomplete, as it didn't update the modalias_show function to generate the
of: modalias string if available.

Fixes: 3ce6c9e2

 ("spi: add of_device_uevent_modalias support")
Signed-off-by: Andreas Schwab <schwab@suse.de>
Link: https://lore.kernel.org/r/mvmwnpi4fya.fsf@suse.de
Signed-off-by: Mark Brown <broonie@kernel.org>