staging: vt6655: rxtx: Replace typedef struct tagSMICHDRHead

With struct vnt_mic_hdr replacing pointer assigments in
s_vFillTxKey.

The size of new struture is the same as old with packing

Create the struture rxtx.h where it is only used.

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