if_oce: fix epoch handling

Thanks to gallatin@ for suggesting the patch.

PR: 260330
Reported by: Vincent Milum Jr.
Reviewed by: gallatin, glebius
Tested by: Vincent Milum Jr.
Differential Revision: https://reviews.freebsd.org/D33395

(cherry picked from commit e363f832cfb283a543fa8e47ebd9e51817994fde)