watchdog: pcwd_usb: overflow in usb_pcwd_send_command()

We changed "buf" from being an array of 6 chars to being a pointer this
sizeof(buf) needs to be updated as well.

Fixes: 2ddb8089a7e5 ('watchdog: pcwd_usb: Use allocated buffer for usb_control_msg')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>