usb_control_msg() must use a dma-capable buffer.

This fixes the following error reported by smatch:

drivers/watchdog/pcwd_usb.c:257 usb_pcwd_send_command() error: doing dma on the
stack (buf)
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>