mfd: twl6040: 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 <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/twl6040.c

···
 	if (twl6040->rev < 0) {
 		dev_err(&client->dev, "Failed to read revision register: %d\n",
 			twl6040->rev);
+		ret = twl6040->rev;
 		goto gpio_err;
 	}