selftests: watchdog: Validate optional file argument

The newly added optional file argument does not validate if the
file is indeed a watchdog, e.g.:

./watchdog-test  -f /dev/zero
Watchdog Ticking Away!

Fix it by confirming that the WDIOC_GETSUPPORT ioctl succeeds.

Fixes: a4864a33 ("selftests: watchdog: Add optional file argument")
Reported-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: George G. Davis <george_davis@mentor.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>