UBI: prepare debugging stuff to further debugfs conversion

We'll need the 'struct ubi_device *ubi' pointer in every debugging function (to
access the ->dbg field), so add this pointer to all the functions implementing
UBI debugging test modes like 'ubi_dbg_is_bitflip()' etc.

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