UBI: fix sparse errors in ubi.h

In C, signed 1-bit bitfields can only take the values 0 and -1, only 0 and 1
are ever assigned in current code.  Make them unsigned bitfields.

Fixes the (repeated) sparse errors:
drivers/mtd/ubi/ubi.h:220:15: error: dubious one-bit signed bitfield
drivers/mtd/ubi/ubi.h:221:17: error: dubious one-bit signed bitfield
drivers/mtd/ubi/ubi.h:222:18: error: dubious one-bit signed bitfield
drivers/mtd/ubi/ubi.h:223:16: error: dubious one-bit signed bitfield
drivers/mtd/ubi/ubi.h:224:20: error: dubious one-bit signed bitfield

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Artem Bityutskiy <dedekind@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>