esp.c: consolidate async_len and TC == 0 checks in do_dma_pdma_cb() and esp_do_dma()

esp.c: consolidate async_len and TC == 0 checks in do_dma_pdma_cb() and esp_do_dma()

Ensure that the async_len checks for requesting data from the SCSI layer and
the TC == 0 checks to detect the end of the DMA transfer are consistent in both
do_dma_pdma_cb() and esp_do_dma(). In particular this involves adding the check
to see if the FIFO is at its low threshold since PDMA and mixed DMA and non-DMA
requests can leave data remaining in the FIFO.

At the same time update all the comments so that they are also consistent between
all similar code paths.

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: <20240112125420.514425-29-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>