net/aquantia-atlantic-kmod: Fix build on systems with hidden struct ifnet

The change to hide 'struct ifnet' (related to the IfAPI) causes a build failure
for this port/driver on FreeBSD 15 and later.
This patch updates the code to use the new IfAPI, resolving the build issue,
but I do not have the hardware to verify functionality.

PR: 282773
Reviewed by: jhibbits