mtd: cmdlinepart: skip partitions truncated to zero

Perform flash size truncation before skipping zero sized partition
so that if the result is a zero sized, it will be skipped like the
others.

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>