staging: vt6656: Remove unnecesssary NULL check

pFifoHead points to tx_buffer->fifo_head which can never be NULL.
We also don't need to check for tx_buffer being NULL instead, since it
always points to ->Data of struct vnt_usb_send_context - the pointer to
which was checked before calling s_vGenerateTxParameter().

Silences a dereference before NULL check warning reported by the coverity
scanner in CID 1127221.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>