media: dw9714: Update to SPDX license identifier

Remove the GPL v2 license boilerplate and update with the SPDX license
identifier.

Signed-off-by: Rajmohan Mani <rajmohan.mani@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>