ti-st: handle null allocation return correctly.

ti-st: handle null allocation return correctly.

static analysis with smatch picked up the following error:

get_platform_data() error: potential null dereference 'dt_pdata'.
  (kzalloc returns null)

Instead, the code should return NULL to avoid the following null
pointer deference.  Also, remove the error message as it is
redundant, the caller emits an error message to alert of a
failure anyhow.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>