drivers: gnss: lcx7g: Close pipe on suspend

The pipe should be closed when suspended, both to save ressources
and to flush it. Without flushing the pipe, the driver may fail
to either resume or suspend the GNSS as the chat module starts
processing old data, which can contain acks to commands, breaking
the scripts.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>