linux: fix a regression in how enslaved devices' MAC addresses are retrieved

linux: fix a regression in how enslaved devices' MAC addresses are retrieved

The MAC address of an enslaved device was not retrieved
correctly. `iflinux_get_permanent_mac()` was called with the master
device instead of the enslaved device. Most of the time, no warning
was issued except when the master device was a slave of another
device (for example, a bridge). This closes #45.