kernel: do not accept sysrq characters via serial port

many boards have a disconnected TTL level serial which can generate
some garbage that can lead to spurious false sysrq detects.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Backport of r37210

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@38863 3c298f89-4303-0410-b956-a3cf2f4a3e73