spi: spi-ep93xx: remove 'dss' from per chip private data

This value is only needed to set the bits per word for each transfer
of a message. There is no reason to set the value in ep93xx_spi_enable()
because ep93xx_spi_process_transfer() sets it again for each transfer.

Just pass the t->bits_per_word directly to ep93xx_spi_chip_setup() in
ep93xx_spi_process_transfer() and remove 'dss' from the per chip private
data.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Mark Brown <broonie@linaro.org>