shell: mqtt: optimize module-scoped `sh_mqtt` variable access

Introduce a local pointer, `struct shell_mqtt *sh`, to minimize access
to the module-scoped `sh_mqtt` variable.
This allows `sh` to be stored in a CPU register, resulting in more
compact code and improved execution efficiency.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>