broadband-modem-qmi: ensure we do not re-use old EPS registration state

When updating the registration state, in the case where LTE is not ready,
ensure we reset the EPS registration state rather than leaving its old
value.
Else when the consolidated registration is built, it can wrongly think
we are registered, e.g. the following case was happening:
[modem0] consolidated registration state: cs 'home', ps 'home', eps 'home', 5gs 'unknown' --> 'home'
a bit later
[modem0] consolidated registration state: cs 'unknown', ps 'unknown', eps 'home', 5gs 'unknown' --> 'home'
then it wrongly tries immediatly to connect
and fails due to 'no-service'

On Qualcomm SC7180, running the following sequence would often reproduce
it:
<stop higher level network manager>
mmcli -m 0 -d
mmcli -m 0 --set-power-state-low
sleep 10
mmcli -m 0 -e
mmcli -m 0 --simple-connect="apn=broadband"

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