drivers: spi: mcux_flexcomm: fix chip select bug w/ dma transfers

Fix for bug:
https://github.com/zephyrproject-rtos/zephyr/issues/59575



The dma version of the version of the driver can
invoke multiple intermediate dma transfers, like
when the spi_buf_set count is greater than one.
However, there is a bug where chip select is not kept
asserted for all intermediate dma transfers required
to process the entire spi_buf_set.

Signed-off-by: Mike J. Chen <mjchen@google.com>