spi: a3700: Change SPI mode before asserting chip-select

The spi device mode should be configured in the controller before the
chip-select is asserted, so that a clock polarity configuration change
is not interpreted as a clock tick by the device.

This patch moves the mode setting to the 'prepare_message' function
instead of the 'transfer_one' function.

By doing so, this patch also removes redundant code in
a3700_spi_clock_set.

This was tested on EspressoBin board, with spidev.

Signed-off-by: Maxime Chevallier <maxime.chevallier@smile.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>