mtd: cmdlinepart: fix skipping zero sized partition

Decrement index i after skipping a zero sized partition.  On next loop
iteration, the index will be the same as before, but the data will be
new as it was moved when earlier partition was skipped.

Signed-off-by: Christopher Cordahi <christophercordahi@nanometrics.ca>
Acked-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>