selftests: watchdog: fix message when /dev/watchdog open fails

When /dev/watchdog open fails, watchdog exits with "watchdog not enabled"
message. This is incorrect when open fails due to insufficient privilege.

Fix message to clearly state the reason when open fails with EACCESS when
a non-root user runs it.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>