Make dev_printk usable from non-GPL modules again

dev_printk now calls dev_driver_string.  We want even proprietary modules
to be calling dev_printk, so the export of dev_driver_string needs to be
non-GPL-only.
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>