mtd: backport upstream bug fix

mtd: backport upstream bug fix

A regression in the mtd package prevents non-consecutive ubi volume
IDs from working properly. This change backported from upstream
reverts the commit [0]. The issue was originally described on the mtd
mailing list:

http://lists.infradead.org/pipermail/linux-mtd/2018-June/081562.html

[0] http://git.infradead.org/mtd-utils.git/commitdiff/dede98ffb706676309488d7cc660f569548d5930

Signed-off-by: David Owens <david.owens@rockwellcollins.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>