USB: gadget: Use ERR_PTR/IS_ERR

Use ERR_PTR and IS_ERR rather than mixing integers and pointers.

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

)

// <smpl>
@@
expression *E;
@@

* E < 0
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>