xlnx_dp: fix iffy xlnx_dp_aux_push_tx_fifo

xlnx_dp: fix iffy xlnx_dp_aux_push_tx_fifo

xlnx_dp_aux_push_tx_fifo takes an immediate uint8_t and a buffer length,
which must be 1 because that is how many uint8_t's fit in a uint8_t.
Sure enough, that is what xlnx_dp_write passes to it, but the function
is just weird.  Therefore, make xlnx_dp_aux_push_tx_fifo look like
xlnx_dp_aux_push_rx_fifo, taking a pointer to the buffer.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>