mtd: spear_smi: failure test for null rather than negative integer

dev_get_platdata returns a pointer, so the failure value would be NULL
rather than a negative integer.

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

)

// <smpl>
@@
expression x,e;
statement S1,S2;
@@

*x = dev_get_platdata(...)
... when != x = e
*if (x < 0) S1 else S2
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>