shell: Fix log message queue size for all backends

188d2dfc introduced a change where log message queue again
become configurable through Kconfig instead of being fixed in
the code. However, it updated only the configuration of an UART
backend. This commit updates other backends as well. Including
dummy backend which has fixed in the code value.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>