mgmt: smp: Fix shell sending SMP response to wrong UART

The commit fixes the problem where the SMP over shell responses have
been sent with use of k_str_out(), which is configured to use console
UART directly.  This caused problem when user decided to attach
shell to a different UART then the one that has been used by
the console driver, because the SMP responses would still be sent
to the console UART rather than the shell UART.

The updated code uses, for shell SMP responses, the UART that has been
selected for the shell.

Fixes #26939

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>