subsys/modem/backend: Correct async UART backend close

The backend currently returns the pipe closed event immediately
after calling uart_rx_disable() which is not the correct behavior.
the pipe closed event should be called when the UART_RX_DISABLED
event is raised by the UART driver.

With this fix, back-to-back open/close/open... will work as
expected, where before the second open would often fail since
the UART was not actually disabled yet.

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