Uninitialization of uart transport was missing disabling of TX
interrupt. It had to be done by the user before using uart.
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
shell_uart_release command shows how shell can be uninitialized and
release transport resources (uart) and reinitialized at runtime.
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>