OMAP: PM: SmartReflex: Add missing IS_ERR test

Function _sr_lookup, defined in the same file, returns ERR_PTR not NULL in
an error case.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@r@
identifier f;
@@
f(...) { ... return ERR_PTR(...); }

@@
identifier r.f, fld;
expression x;
statement S1,S2;
@@
 x = f(...)
 ... when != IS_ERR(x)
(
 if (IS_ERR(x) ||...) S1 else S2
|
*x->fld
)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Kevin Hilman <khilman@ti.com>