if_genet: Consistently use the boolean form

The variable eaddr_found is declared as bool. This change improves
readability a little.

No functional change intended.

MFC after: 3 days

(cherry picked from commit 2fc051d090933203e1af6a8452cdd87a4c76ad5b)
(cherry picked from commit 97d5f16c6966e2ae261ddabe1f16a9675dedf43a)