[MTD] MTD_DEBUG always does compile-time typechecks

The current style for debug messages is to ensure they're always
parsed by the compiler and then subjected to dead code removal.
That way builds won't break only when debug options get enabled,
which is common when they are stripped out early by CPP.

This patch makes CONFIG_MTD_DEBUG adopt that convention.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>