watchdog: Fix POST failure on ASUS P5N32-E SLI and similar boards

At present the module does not unset the NO_REBOOT bit upon shutdown, this
causes the BIOS to fail the POST once and reset. During the next boot it
displays the following error message:

***** Warning: System BOOT Fail *****
Your system last boot fail or POST interrupted.
Please enter setup to load default and reboot again.
Press F1 to continue, DEL to enter SETUP

With this patch the NO_REBOOT flag will be unset on shutdown and thus stop
this failure from occurring.

Tested on 'ASUS P5N32-E SLI with BIOS revision 1801' and
          'ASUS P5N32-E SLI PLUS with BIOS revision 1502'.

Signed-off-by: Mart Gerrits <mart1987@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>