selftests: watchdog: Fix error message.

[ Upstream commit 04d5e4bd37516ad60854eb74592c7dbddd75d277 ]

Printf's say errno but print the string version of error.
Make consistent.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>