linux-postmarketos-qcom-{sc7280,sm6350}: enable CONFIG_RMNET
Required for getting mobile data up, otherwise ModemManager fails to
connect with:
Status | connected: no
| connection error name: org.freedesktop.ModemManager1.Error.MobileEquipment.Unknown
| connection error message: failed to create net link for device: failed to add link for device: Could not allocate link: Failed to add link with mux id 1: Netlink message with transaction 27 failed