The current way of looking up the driver name using the sysfs and more specifically the 'module' file, does not work if the driver is not a module.
If the driver is compiled into the kernel directly 'module' does not exist.

This issue is commonly seen when running custom kernels that include certain device drivers with =y instead of =m.
By leveraging the output of 'ethtool --driver <iface>' the driver can be reliably detected in these circumstances too.

Regression tested on x86 for igc and arm64 qemu virtio-net with CONFIG_VIRTIO_NET=y set.