mtd: Replace DEBUG with MTD_DBG

[Tanmay Upadhyay]

I came across this when I got compilation errors in USB host driver. I found
the errors were because of 'DEBUG' macro in 'mtd.h'. 'mtd.h' was included in
SoC specific common header. Unlike MTD, USB subsystem expects DEBUG to be a
boolean - be defined or undefined - to on/off DEBUG code. I guess this is
common practice in other subsystems as well.

To solve this I thought to replace DEBUG to with MTD_DBG. I am not sure if
this is a proper way to do this or not. I am more than willing to change
if this can be achieved in a better way.

Thanks,
Tanmay

PS: This patch involves changes in some 'quite old' files, which has code
indented with spaces instead of tab. So, checkpatch.pl throws white space
errors for a few files.