USB: gadget code switches to pr_err() and friends

We now have pr_err(), pr_warning(), and friends ... start using
them in the gadget stack instead of printk(KERN_ERR) and friends.
This gives us shorter lines and somewhat increased readability.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>