[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.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>