spi: docs: Use the multi-lane mode constants in examples

The multi-lane documentation names SPI_MULTI_BUS_MODE_* constants,
including in the transfer initializers. The interface introduced with
the document uses SPI_MULTI_LANE_MODE_*, so the examples refer to
undefined macros.

Use the interface's constants in the descriptions and examples.

Fixes: 05c3bd745bb0 ("spi: Documentation: add page on multi-lane support")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260910033219.15996-1-kmehltretter@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>