mfd: tps6594: Use spi_get_chipselect() API to access spi->chip_select

mfd: tps6594: Use spi_get_chipselect() API to access spi->chip_select

In preparation for adding multiple CS support for a device, set/get
functions were introduces accessing spi->chip_select in
'commit 303feb3cc06a ("spi: Add APIs in spi core to set/get
spi->chip_select and spi->cs_gpiod")'.
Replace spi->chip_select with spi_get_chipselect() API.

Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@amd.com>
Link: https://lore.kernel.org/r/20231125092137.2948-2-amit.kumar-mahapatra@amd.com
Signed-off-by: Lee Jones <lee@kernel.org>