fsl_ssi: fix of_property_read_u32_array return value check

of_property_read_u32_array returns 0 on success,
so the return value shouldn't be inverted twice,
first on assignment then in condition expression.

Signed-off-by: Maciej Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>