net: ipa: only set endpoint netdev pointer when in use

In ipa_modem_start(), we set endpoint netdev pointers before the
network device is registered.  If registration fails, we don't undo
those assignments.  Instead, wait to assign the netdev pointer until
after registration succeeds.

Set these endpoint netdev pointers to NULL in ipa_modem_stop()
before unregistering the network device.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>