Return `ret` in op_ifentry(..) to mute a -Wunused-but-set-variable warning

Return `ret` in op_ifentry(..) to mute a -Wunused-but-set-variable warning

This will also now detect error conditions with
value->var.subs[sub - 1] == LEAF_ifPhysAddress where `string_get(..)`
could fail if iifp->physaddr and/or iifp->physaddrlen were deemed
invalid.

MFC after: 2 weeks
CID: 1006551
Reported by: Coverity, gcc
Sponsored by: EMC / Isilon Storage Division