NAND: Rename DEBUG to MTDDEBUG to avoid namespace pollution.

This is particularly problematic now that non-NAND-specific code is
including <nand.h>, and thus all debugging code is being compiled
regardless of whether it was requested, as reported by Scott McNutt
<smcnutt@psyent.com>.

Signed-off-by: Scott Wood <scottwood@freescale.com>