modem: Delegate async UART pipe closed event

This commit delegates the modem_pipe_notify_closed() call
resulting from the UART async API UART_RX_DISABLED event
to the workqueue. This is neccesary as the async UART
callback may be called from ISR context.

modem_pipe_notify_closed() must be called from outside of
the ISR context as it takes a mutex.

The commit also adds a missing break to the async UART
callback, and adds a missing dependency to the Kconfig
for the UART backends, RING_BUFFER=y

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>