asix-sigma: fix build time warnings

asix-sigma: fix build time warnings

We need to include unistd.h for usleep():
hardware/asix-sigma/asix-sigma.c: In function 'sigma_fpga_init_bitbang':
hardware/asix-sigma/asix-sigma.c:450:3: warning: implicit declaration of function 'usleep'
   usleep(10000);

And we need to tweak the type of chunks_per_read to match dl_lines_total:
hardware/asix-sigma/asix-sigma.c: In function 'download_capture':
hardware/asix-sigma/asix-sigma.c:1161:39: warning: comparison between signed and unsigned integer expressions
   dl_lines_curr = MIN(chunks_per_read, dl_lines_total);