treewide: add space before SPDX identifier

treewide: add space before SPDX identifier

Strictly, an SPDX identifier requires a space between the comment
marker and the identifier itself. The choice of the comment marker
itself is irrelevant.

Correct:

// SPDX-License-Identifier: GPL-2.0-or-later OR MIT

Wrong:

//SPDX-License-Identifier: GPL-2.0-or-later OR MIT

Fix that in the whole tree (actually, only ramips contained wrong
uses).

Found by checkpatch.pl

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>