Input: sysrq - fix "stuck" SysRq mode

This shoud fix the problem with SysRq mode staying half-way enabled
and interfereing with normal PrtScrn operation after user presses ALT
for the first time.

Reported-and-tested-by: Éric Piel <E.A.B.Piel@tudelft.nl>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>