spi: stm32: split large transfers based on word size instead of bytes

The TSIZE register in CR2, to which the number of words to transfer
is written, is only 16 Bit. This limits transfers to 65535 SPI
_words_ at a time. The existing code uses spi_split_transfers_maxsize
to limit transfers to 65535 _bytes_ at a time.

This breaks large transfers with bits_per_word > 8, as they are
split inside of a word boundary by the odd size limit.

Split transfers based on the number of words instead.
This has the added benefit of not artificially limiting the maximum
length of bpw > 8 transfers to half or a quarter of the actual limit.

The combination of very large transfers and bits_per_word = 16 is triggered
e.g. by MIPI DBI displays when updating large parts of the screen.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Acked-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/20230310092053.1006459-2-l.goehrs@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>