mfd: mxs-lradc: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/mfd/mxs-lradc.c: In function ‘mxs_lradc_probe’:
drivers/mfd/mxs-lradc.c:179:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (lradc->soc == IMX28_LRADC) {
       ^
drivers/mfd/mxs-lradc.c:185:3: note: here
   default:
   ^~~~~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>