spi: fix misleading controller registration kernel-doc

spi: fix misleading controller registration kernel-doc

The controller reference count is not decremented on registration
failures (and has not been for a very long time) contrary to what the
outdated kernel-doc says.

Drop the entire sentence about return values which are now documented in
the Return section.

Fixes: b885244eb262 ("[PATCH] spi: add spi_driver to SPI framework")
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20260312151817.32100-5-johan@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>