fastboot: Remove '\n' from interface name on linux

If the device provides an interface string, we should remove the newline
from the read file that is added from by the linux kernel.

Bug: 324320178
Test: Ran the same command with my local changes vs not

```
bash$ ./fastboot devices -l
5f42ad5ad259c90cf14ea222791b6aaa fastboot usb:7-3

bash$ fastboot devices -l
5f42ad5ad259c90cf14ea222791b6aaa fastboot
 usb:7-3
```

Change-Id: Ida3316fdba8e35f0c66784f83455a4d82e90ba1c