mfd: ab8500-core: Return zero in get_register_interruptible()

This function is supposed to return zero on success or negative error
codes on error. Unfortunately, there is a bug so it sometimes returns
non-zero, positive numbers on success.

I noticed this bug during review and I can't test it. It does appear
that the return is sometimes propogated back to _regmap_read() where all
non-zero returns are treated as failure so this may affect run time.

Fixes: 47c1697508f2 ("mfd: Align ab8500 with the abx500 interface")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/ab8500-core.c

···
 	mutex_unlock(&ab8500->lock);
 	dev_vdbg(ab8500->dev, "rd: addr %#x => data %#x\n", addr, ret);
 
-	return ret;
+	return (ret < 0) ? ret : 0;
 }
 
 static int ab8500_get_register(struct device *dev, u8 bank,