staging: vt6655: Replace typedef struct tagSTxDesc

Replace with struct vnt_tx_desc with all members the same.

volatile is removed from pointers as this generates warning
message.

Only the first four members of vnt_tx_desc need to be volatile.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>