UBI: rename a local variable

Rename local variable 'ec_corr' into 'ec_err' to make the code a little bit
more readable. 'ec_err' is more appropriate because it sounds more like 'error
when EC was read' and it looks more logical because we use it together with
'err'. Just a minor nicification which should improve the rather complex
scanning code.

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