asix-sigma: Rework download_capture() to support more than 32 lines

asix-sigma: Rework download_capture() to support more than 32 lines

In the current configuration, the download capture used 32KiB buffer
for samples. This was the upper limit this function could download
from the Sigma. Even the sigma_read_dram() was only called once to
read up-to 32 DRAM line from address 0x0 in the DRAM.

This patch reworks the function to call sigma_read_dram() in a loop
in case there is need to download more than 32 DRAM lines of data
from Sigma. The data are then correctly passed for decoding to the
decoding function.

Signed-off-by: Marek Vasut <redacted>