esp: apply transfer length adjustment when STC is zero at TC load time

esp: apply transfer length adjustment when STC is zero at TC load time

Perform the length adjustment whereby a value of 0 in the STC represents
a transfer length of 0x10000 at the point where the TC is loaded at the
start of a DMA command rather than just when a TI (Transfer Information)
command is executed. This better matches the description as given in the
datasheet.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210304221103.6369-12-mark.cave-ayland@ilande.co.uk>