UBI: gluebi: initialize ubi_num field

Do not forget to initialize 'gluebi->ubi_num' because otherwise
it will stay 0 even for ubi1 device, and gluebi will open
wrong UBI device when 'gluebi_get_device()' is called.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>