scsi: esp: clean up handle_ti/esp_do_dma if s->do_cmd

Avoid duplicated code between esp_do_dma and handle_ti.  esp_do_dma
has the same code that handle_ti contains after the call to esp_do_dma;
but the code in handle_ti is never reached because it is in an "else if".
Remove the else and also the pointless return.

esp_do_dma also has a partially dead assignment of the to_device
variable.  Sink it to the point where it's actually used.

Finally, assert that the other caller of esp_do_dma (esp_transfer_data)
only transfers data and not a command.  This is true because get_cmd
cancels the old request synchronously before its caller handle_satn_stop
sets do_cmd to 1.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>