trivial: modem-manager: chomp vendor string
trivial: modem-manager: chomp vendor string

Reading the sysfs file seemed to have also eaten the `\n` as mentioned
on a bug.

```
├DW5821e Snapdragon X20 LTE:
│     Device ID:           fa707b9af86ff44bc17316b6c3e5ea82aab3ce86
│     Summary:             Mobile broadband device
│     Current version:     T77W968.F1.0.0.4.2.GC.010
│     Vendor:              Dell Inc. (USB:0x413c
│     )
│     GUIDs:               64da2d58-8d1b-5e5b-b793-f88ba5a25a8f
│                          761d6124-0002-5185-b767-9adf67bf1a5e
│                          795e079d-093b-5503-aa59-35b832480e95
│     Device Flags:        • Updatable
```

fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd