esp.c: update esp_do_dma() bypass if async_len is zero to include non-zero transfer...

esp.c: update esp_do_dma() bypass if async_len is zero to include non-zero transfer check

In the PDMA case the last transfer from the device to the FIFO has occurred
(async_len is zero) but esp_do_dma() is still being called to drain the
remaining FIFO contents.

The additional non-zero transfer check ensures that we still defer the SCSI
layer in the case where we are waiting for data for a TI command or a DMA
enable signal.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Helge Deller <deller@gmx.de>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20240112125420.514425-35-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>