ASoC: fsl_ssi: properly initialize the sysfs attribute object

Commit 6992f533 ("sysfs: Use one lockdep class per sysfs attribute")
requires 'struct attribute' objects to be initialized with sysfs_attr_init().

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org