staging, media, easycap: Fix mem leak in easycap_usb_probe()
If allocating 'pdata_urb' fails, the function will return -ENOMEM
without freeing the memory allocated, just a few lines above, for
'purb' and will leak that memory when 'purb' goes out of scope.
This patch resolves the leak by freeing the allocated storage with
usb_free_urb() before the return.