spi: spi-fsl-dspi: Fix continuous selection format

Current DMA implementation was not handling the continuous selection
format viz. SPI chip select would be deasserted even between sequential
serial transfers.

Use existing dspi_data_to_pushr function to restructure the transmit
code path and set or reset the CONT bit on same lines as code path
in EOQ mode does. This correctly implements continuous selection format
while also correcting and cleaning up the transmit code path.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>