asix-sigma: Remove stack-based alloc in download_capture()

asix-sigma: Remove stack-based alloc in download_capture()

Avoid allocating 32KiB of data on stack in download_capture(). Instead,
do a glib-variant of calloc(1, ) to allocate the data for the samples
which will be downloaded. This avoids explosions of stack on systems
with tight stack limits.

Furthermore, define structures describing the organisation of Sigma's
DRAM memory and start using those instead of ad-hoc preprocessor macros
defining the sizes of various structures in memory.

Signed-off-by: Marek Vasut <redacted>