spi: atmel: factorize reusable code for SPI controller init

The SPI controller configuration during the init can be reused, for the
resume function for example.

Let's move this configuration to a separate function.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>