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

This patch fixes the following warnings:

drivers/mtd/devices/phram.c: In function ‘parse_num64’:
drivers/mtd/devices/phram.c:149:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
     shift += 10;
     ~~~~~~^~~~~
drivers/mtd/devices/phram.c:150:4: note: here
    case 'M':
    ^~~~
drivers/mtd/devices/phram.c:151:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
     shift += 10;
     ~~~~~~^~~~~
drivers/mtd/devices/phram.c:152:4: note: here
    case 'k':
    ^~~~

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

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Richard Weinberger <richard@nod.at>