drivers: serial: nrfx: Skip poll out when device is not active

Added early return from uart_poll_out when uart device is not in
active state. Poll_out is used from many contexts by multiple users
and currently here is no mechanism in power management to disable
them (console, logger when going to inactive power state.

Additionally, moved setting new state when disactivating the device to
ensure that no poll_out will happen after disabling the UARTE.

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