staging: vt6656: vnt_fill_cts_head remove unneeded NULL check.

union vnt_tx_data_head is nolonger detached from main
vnt_tx_buffer structure so this check is unnecessary.

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