spi/bcm63xx: properly prepare clocks before enabling them

Use proper clk_prepare/unprepare calls in preparation for switching
to the generic clock framework.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>