staging: ralink-gdma: add proper SPDX identifiers on ralink-gdma file

The ralink-gdma.c driver did not have a SPDX identifier on it, so fix
that up.  At the same time, remove the "free form" text that specified
the license of the file, as that is impossible for any tool to properly
parse.

Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>