uqmi: set plmn only if necessary

uqmi: set plmn only if necessary

Setting the plmn to '0' (auto) will implicitly lead to a (delayed)
network re-registration, which could further lead to some timing
related issues in the qmi proto handler.

On the other hand, if you switch back from manual plmn selection
to auto mode you have to set it to '0', because this setting is
permanently "saved" in the wwan module.

Conclusion:
If plmn is configured, check if it's already set euqally in the module.
If so, do nothing. Otherwise set it.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>