shell: fix resize command

Resize command requires shell to be working on UART interrupt
driven API. This will be now enabled by default when shell serial
backend is selected. Recently this feature was accidentally turned
off in PR #11556.

k_sleep(1) has been replaced with k_busy_wait(1000) to avoid
hanging in command handler.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>