Disabling gpe18 at reboot

[Joe Riel <joer@san.rr.com>]

Following an upgrade to Wheezy, my Lenovo Y560p laptop
showed 80% cpu usage on one core.  This was due to 
constant interrupts on gpe18, see, udoremember.blogspot.com.

I can stop this by executing

sudo echo disable > /sys/firmware/acpi/interrupts/gpe18

I'd like to have that executed at reboot.  To do so,
I added the following cron file:

# /etc/cron.d/30-disable-gpe18
@reboot root echo disable > /sys/firmware/acpi/interrupts/gpe18

Alas, that does not work.  I've verified that the line is executed,
however, the interrupt is not disabled.  Any ideas why that would
be the case?  

-- 
Joe Riel