staging: wfx: explain uncommon Makefile statement

I have got questions about this line from several reviewers. A comment
is definitively welcome.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20220113085524.1110708-6-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/Makefile

@@ -20,6 +20,7 @@ wfx-y := \
 	sta.o \
 	debug.o
 wfx-$(CONFIG_SPI) += bus_spi.o
+# When CONFIG_MMC == m, append to 'wfx-y' (and not to 'wfx-m')
 wfx-$(subst m,y,$(CONFIG_MMC)) += bus_sdio.o
 
 obj-$(CONFIG_WFX) += wfx.o