UBI: fix IS_ERR test

In case of error, the function add_volume returns an ERR pointer. The
result of IS_ERR, which is supposed to be used in a test as it is, is
here checked to be less than zero, which seems odd. We suggest to
replace this test by a simple IS_ERR test.

A simplified version of the semantic match that finds this problem is
as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@def0@
expression x;
position p0;
@@
x@p0 = add_volume(...)

@protected@
expression def0.x,E;
position def0.p0;
position p;
statement S;
@@
x@p0
... when != x = E
if (!IS_ERR(x) && ...) {<... x@p ...>} else S

@unprotected@
expression def0.x,E;
identifier fld;
position def0.p0;
position p != protected.p;
@@
x@p0
... when != x = E
* x@p->fld
// </smpl>

Signed-off-by: Julien Brunel <brunel@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>