ASoC: samsung: i2s: Prevent potential NULL platform data dereference

When np is NULL i2s_pdata could also be NULL but i2s_pdata is now being
dereferenced without proper check. Fix this and shorten the error message
so we don't exceed 80 characters limit.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>