nas: add --get-plmn

nas: add --get-plmn

This command is needed in the qmi proto handler to check if the plmn
is already set to 'auto'.

The reason for this is, that setting the plmn to 'auto' will implicitly
lead to a (delayed) network re-registration, which could further lead
to some timing related issues in the qmi proto handler.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>