staging: vt6656: fix potential NULL pointer dereference

vnt_free_tx_bufs() relies on priv->tx_context elements to be NULL if
they are not initialized (as vnt_free_rx_bufs() does). Add a check to
these elements in order to avoid NULL pointer dereference.

Signed-off-by: Quentin Deslandes <quentin.deslandes@itdev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>