ANDROID: usb: gadget: f_mtp: simplify ptp NULL pointer check

Simplify MTP/PTP dev NULL pointer check introduced in
Change-Id: Ic44a699d96df2e13467fc081bff88b97dcc5afb2
and restrict it to MTP/PTP function level only.

Return ERR_PTR() instead of NULL from mtp_ptp function
to skip doing NULL pointer checks all the way up to
configfs.c

Fixes: Change-Id: Ic44a699d96df2e13467fc081bff88b97dcc5afb2
       ("usb: gadget: fix NULL ptr derefer while symlinking PTP func")
Change-Id: Iab7c55089c115550c3506f6cca960a07ae52713d
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

drivers/usb/gadget/configfs.c

@@ -427,11 +427,6 @@ static int config_usb_cfg_link(
 	}
 
 	f = usb_get_function(fi);
-	if (f == NULL) {
-		/* Are we trying to symlink PTP without MTP function? */
-		ret = -EINVAL; /* Invalid Configuration */
-		goto out;
-	}
 	if (IS_ERR(f)) {
 		ret = PTR_ERR(f);
 		goto out;

drivers/usb/gadget/function/f_mtp.c

@@ -1497,7 +1497,7 @@ struct usb_function *function_alloc_mtp_ptp(struct usb_function_instance *fi,
 		pr_err("\t2: Create MTP function\n");
 		pr_err("\t3: Create and symlink PTP function"
 				" with a gadget configuration\n");
-		return NULL;
+		return ERR_PTR(-EINVAL); /* Invalid Configuration */
 	}
 
 	dev = fi_mtp->dev;

drivers/usb/gadget/functions.c

@@ -58,7 +58,7 @@ struct usb_function *usb_get_function(struct usb_function_instance *fi)
 	struct usb_function *f;
 
 	f = fi->fd->alloc_func(fi);
-	if ((f == NULL) || IS_ERR(f))
+	if (IS_ERR(f))
 		return f;
 	f->fi = fi;
 	return f;