media: siano: add SPDX markups

As we're now using SPDX identifiers, add the proper SPDX,
better identifying the licenses whith apply to the source code.

As we're now using the short license, it doesn't make sense to
keep the original license text.

Also, fix MODULE_LICENSE to properly identify GPL v2
at the Siano's common driver. Some codes there are licensed
on GPL v2 or latter, while others are GPL v2 only. So,
in order to reflect the common license that applies to
everything, the module itself should be GPLv2 only.

While here, use the Kernel's coding style for the comments
with copyright info.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>