ubiformat: fix the subpage size hint on the error path

ubiformat: fix the subpage size hint on the error path

David Binderman <dcb314@hotmail.com> reports that the following piece of looks
wrong:

if (!args.subpage_size != mtd->min_io_size)
    normsg("may be sub-page size is incorrect?");

I totally agree with him and I believe that we actually meant to have no
negation in fron to f 'args.subpage_size', so instead, the code should look
like this:

if (args.subpage_size != mtd->min_io_size)
    normsg("may be sub-page size is incorrect?");

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>