Staging: csr: provide a MODULE_LICENSE for csr_helper.ko

I forgot a license for the csr_helper module, so it would not properly
load as it depends on other GPL-only symbols.

Reported-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Cc: Mikko Virkkilä <mikko.virkkila@bluegiga.com>
Cc: Riku Mettälä <riku.mettala@bluegiga.com>
Cc: Veli-Pekka Peltola <veli-pekka.peltola@bluegiga.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>