modem: backend: uart_async: Use single ring buffer

Use single ring buffer and protect it with a spinlock as it is
shared between backend and UART thread (ISR). This is simpler
than the double ring buffer setup.

The receive idle timeout has also been made configurable
instead of being a hardcoded value.

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