staging: vt6656: Fix vnt_tx_usb_header static checker warning

staging: vt6656: Fix vnt_tx_usb_header static checker warning

drivers/staging/vt6656/rxtx.c:729 vnt_beacon_xmit()
warn: struct type mismatch 'vnt_beacon_buffer vs vnt_tx_usb_header'

Since the only part of vnt_beacon_buffer is used remove and
replace it with vnt_tx_usb_header.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/9818e564-81f6-a683-caa0-69423fded401@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>