comgt-ncm: do not attempt to connect if the control device is invalid

comgt-ncm: do not attempt to connect if the control device is invalid

After a hardware reconnect, the control device might be unavailable and
attempting to interact with it will lead to hanging gcom calls, leaving
the protocol setup in an unrecoverable state.

Change the protocol handler to bail out early and notify netifd if the
control device is not defined or if the underlying device node does not
exist.

Also ensure that the "disconnect", "connect" and "setmode" commands are
actually defined before trying to invoke them.

Finally attempt to re-query the device manufacturer if it is unset in
the interface state in order to prevent UNUPPORTED_MODEM errors after
a modem hardware reconnect.

Signed-off-by: Rozhuk Ivan <rozhuk.im@gmail.com>
[reword subject and commit message]
Ref: https://github.com/openwrt/openwrt/pull/2352
Signed-off-by: Jo-Philipp Wich <jo@mein.io>