spi: Group cs_change and cs_off flags together in struct spi_transfer

The commit 5e0531f6

 ("spi: Add capability to perform some transfer
with chipselect off") added a new flag but squeezed it into a wrong
group of struct spi_transfer members (note that SPI_NBITS_* are macros
for easier interpretation of the tx_nbits and rx_nbits bitfields).

Group cs_change and cs_off flags together and their doc strings.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220908130518.32186-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>