staging: vt6656: rxtx.c change tx functions to return u16

s_vGenerateTxParameter, s_vFillCTSHead, s_vFillRTSHead, vnt_rxtx_rts* functions
to return u16

Initially this is the duration_id field for ieee80211_hdr

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