spi: keep track of number of chipselects in spi_device

spi: keep track of number of chipselects in spi_device

There are several places where we need to iterate over a device's
chipselect. To be able to do it efficiently, store the number of
chipselects in spi_device, like we do for controllers.

Since we now use a device supplied value, add a check to make sure it
isn't more than we can support.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://patch.msgid.link/20250915183725.219473-3-jonas.gorski@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>