module: rename set_license() to module_license_taint_check()

The set_license() routine would seem to a reader to do some sort of
setting, but it does not. It just adds a taint if the license is
not set or proprietary.

This makes what the code is doing clearer, so much we can remove
the comment about it.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>