selftests: watchdog: fix mixed whitespace

Convert spaces to tabs for checkpatch compliance. Quick way to verify
this is by running `git show -w <commit-id>`, which returns an empty
commit body. No functional change intended.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>