spi: offload: check for match callback when a trigger is being registered

spi: offload: check for match callback when a trigger is being registered

Make match a required callback when a new trigger is being registered,
this allows that other functions like spi_offload_trigger_get() could
safely invoke the callback when it is required

In v2:
- improve readability of the condition

Signed-off-by: Andres Urian Florez <andres.emb.sys@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250418224750.46219-1-andres.emb.sys@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>