spi: ath79: Remove now useless code

The custom setup/cleanup routines included in the ath79 driver only
take care of setting the initial CS state. However that is already
handled by the bitbang code, so this code can be removed.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>