spi: fsl-dspi: Fix CTAR selection

There are only 4 CTAR registers (CTAR0 - CTAR3) so we can only use the
lower 2 bits of the chip select to select a CTAR register.
SPI_PUSHR_CTAS used the lower 3 bits which would result in wrong bit values
if the chip selects 4/5 are used. For those chip selects SPI_CTAR even
calculated offsets of non-existing registers.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org