drivers: serial: async_to_irq: Fix initialization

Callback was set in uart_async_to_irq_rx_enable() which is only called
when RX is enabled. Callback shall be called in the init function.
uart_async_to_irq_init() signature changed to take device as the
input argument instead of data and config for the adaptation layer
(which can be retrieved from the device). Device is needed when
setting a callback during the initialization.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>