spi: Merge spi_controller.{slave,target}_abort()

Mixing SPI slave/target handlers and SPI slave/target controllers using
legacy and modern naming does not work well: there are now two different
callbacks for aborting a slave/target operation, of which only one is
populated, while spi_{slave,target}_abort() check and use only one,
which may be the unpopulated one.

Fix this by merging the slave/target abort callbacks into a single
callback using a union, like is already done for the slave/target flags.

Fixes: b8d3b056

 ("spi: introduce new helpers with using modern naming")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/809c82d54b85dd87ef7ee69fc93016085be85cec.1667555967.git.geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>