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.deSigned-off-by: Mark Brown <broonie@kernel.org>