Specifies if MAC learning is enabled on the interface.
on(1). MAC learning is enabled. The forwarding function
learns what interface to use for a destination by looking at
the source MAC address in received frames.
off(2) MAC learning is disabled.
Regardless of the setting, learning is disabled if
ethFwdMACMode is set to nomac(3) or if it is set to auto(1)
and less than three interfaces are connected to the
forwarding function.