watchdog: cpwd: Fix buffer-overflow

cppcheck-1.47 reports:
[drivers/watchdog/cpwd.c:650]: (error) Buffer access out-of-bounds: p.devs

The source code is
	for (i = 0; i < 4; i++) {
		misc_deregister(&p->devs[i].misc);

where devs is defined as WD_NUMDEVS big and WD_NUMDEVS is equal to 3.
So the 4 should be a 3 or WD_NUMDEVS.

Reported-By: David Binderman
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>