f_rockusb: Avoid use-after-free in the global pointer variable

In case of usb_add_function() failure the error path has two issues:
 - the potentially allocated structure isn't getting freed
 - the global pointer variable is assigned to garbage

Fix the above mentioned issues by freeing memory and assigning NULL.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>