spi: stm32: fix accidential revert to byte-sized transfer splitting

Commit 6f486556 ("spi: stm32: renaming of spi_master into
spi_controller") included an accidential reverted of a change added in
commit 1e492911 ("spi: stm32: split large transfers based on word
size instead of bytes").

This breaks large SPI transfers with word sizes > 8 bits, which are
e.g. common when driving MIPI DBI displays.

Fix this by using `spi_split_transfers_maxwords()` instead of
`spi_split_transfers_maxsize()`.

Fixes: 6f486556

 ("spi: stm32: renaming of spi_master into spi_controller")
Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Link: https://lore.kernel.org/r/20230816145237.3159817-1-l.goehrs@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>