drivers: video: fbdev: atmel_lcdfb.c: fix error return code

Convert a zero return value on error to a negative one, as returned
elsewhere in the function.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/

)

// <smpl>
(
if@p1 (\(ret < 0\|ret != 0\))
 { ... return ret; }
|
ret@p1 = 0
)
... when != ret = e1
    when != &ret
*if(...)
{
  ... when != ret = e2
      when forall
 return ret;
}

// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>