spi/spi-fsl-spi: Make sure in spi_fsl_setup that chipselect becomes inactive

This is needed for a device in SPI_CS_HIGH mode that otherwise could start out
active for the first transaction.

Acked-by: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>