ixgbe: fix impossible condition

Coverity flagged this condition: The condition

offset == 0 && offset == 65535

can never be true because offset cannot be equal
to two different values at the same time.

Submitted by: bret_ketchum@dell.com
Reported by: Coverity
Reviewed by: tsoome, cem
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D26144

(cherry picked from commit ab1c54fec66803235a8923333fa79f2cbfa33354)