ASoC: Blackfin: allow SPI for SSM2602 parts

This board has hardware switches for selecting SPI or I2C, so don't
require I2C for this driver.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>