media: mt9m001: convert to SPDX license identifer

Replace GPL license statements with SPDX license identifiers (GPL-2.0).

This also fixes MODULE_LICENSE() ident to match the actual license text.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>