USB: musb_gadget: implement set_wedge() method

Implement the driver's set_wedge() method by adding the 'wedged' flag
to the 'struct musb_ep'.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>