wireless-regdb: install crda files only if used

Since linux-4.15 the kernel supports loading the files
regulatory.db/regulatory.db.p7s directly from the /lib/firmware
directory, for earlier versions the crda helper application
is needed.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>