sfc/siena: Make MCDI logging support specific for Siena

Add a Siena Kconfig option and use it in stead of the sfc one.
Rename the internal variable for the 'mcdi_logging_default' module
parameter to avoid a naming conflict with the one in sfc.ko.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>