mtd: cmdlinepart: update /proc/mtd comment

/proc/mtd doesn't contain the mtd-id of the device, but the part name from the
command line.  This corrects what I believe is an obsolete comment from commit
a0ee24a0.

Signed-off-by: Christopher Cordahi <christophercordahi@nanometrics.ca>
Cc: Philip Rakity <prakity@yahoo.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>