spi: img-spfi: Set device select bits for SPFI port state

Even if the chip select line is not controlled by the SPFI
hardware, the device select bits need to be set to specify
the chip select line in use for the hardware to know what
parameters to use for the current transfer.

Signed-off-by: Ionela Voinescu <ionela.voinescu@imgtec.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>