shell: log_backend: Fix fifo cleanup
shell: log_backend: Fix fifo cleanup

When shell log backend was disabled fifo used only in
deferred mode was unconditionally flushed which lead to
errors in immediate mode where fifo was not present. On
the other hand, in case of LOG2_MODE_DEFERRED fifo (mpsc_pbuf)
was initialized in enable stage.

In order to clean things up, deferred v1 fifo flushing and
deferred v2 mpsc pbuf initialization were moved to fifo_reset
function which is called when shell log backend is enabled.

Signed-off-by: Krzysztof Chruscinski <9b6dc988558c51b0405b49b55c8e66862969844b@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr