net: ipa: open-code ether_setup()

About half of the fields set by the call in ipa_modem_netdev_setup()
are overwritten after the call.  Instead, just skip the call, and
open-code the (other) assignments it makes to the net_device
structure fields.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>