esp.c: only call dma_memory_read function if transfer length is non-zero

esp.c: only call dma_memory_read function if transfer length is non-zero

In the cases where mixed DMA/non-DMA transfers are used or no data is
available, it is possible for the calculated transfer length to be zero.
Only call the dma_memory_read function where the transfer length is
non-zero to avoid invoking the DMA engine for a zero length transfer
which can have side-effects (along with generating additional tracing
noise).

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250711204636.542964-5-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>