net: if: Check chksum offloading properly for VLAN interfaces

Make sure we check the checksum offloading capabilities correctly
for VLAN interfaces. Use the real Ethernet interface when doing the
check.

Fixes #78724

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
(cherry picked from commit 00bb90a4)