spi: spi-ep93xx: move the clock divider calcs into ep93xx_spi_chip_setup()

The divider values stored in the per chip data are only used to set the
registers in the hardware to generate the desired SPI clock. Since these
are calculated per transfer based on the t->speed_hz there is no reason
keep them in the per chip data.

Move the ep93xx_spi_calc_divisors() call into ep93xx_spi_chip_setup()
and return the dividers thru pointers. Remove the divider values from
the per chip data structure.

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>