Replace short statement in comment with proper SPDX license tag.

Note, for i2c-desingware-slave.c the identifier is chosen
in accordance with MODULE_LICENSE() macro since it is visible to user.
Another point to this choice is that the header seems to be copy'n'paste
from the other file of this very driver.
Acked-by: NLuis Oliveira <Luis.Oliveira@synopsys.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: NJarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>