mgmt: smp: shell: Respond on uart shell device when changed at runtime

Responses are currently set to the shell device that was configured
in the device tree.

The shell_uart driver allows for changing it's device during runtime
which leads to a situation where we recieve packets on one device
and respond on another.

This patch causes smp_shell_tx_raw to use the shell_uart device

Signed-off-by: Zak Portnoy <zakportnoy@gmail.com>