tty/sysrq: constify the the sysrq_key_op(s)

All the users threat them as immutable - annotate them as such.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-3-emil.l.velikov@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>