asix-sigma: comment on trigger/stop position, silence warning

asix-sigma: comment on trigger/stop position, silence warning

Add a comment on the logic which skips the upper 64 bytes of a 512 bytes
chunk in the Asix Sigma's sample memory. Move the initial assignment and
the subsequent update from a value which was retrieved from a hardware
register closer together for awareness during maintenance. Pre-setting a
high position value that will never match when the feature is not in use
is very appropriate.

Adjust the sigma_read_pos() routine to handle triggerpos identically to
stoppos. The test condition's intention is to check whether a decrement
of the position ends up in the meta data section of a chunk. The previous
implementation tested whether a pointer to the position variable ended in
0x1ff when decremented -- which is unrelated to the driver's operation.
It's assumed that no harm was done because the trigger feature is
unsupported (see bug #359).

This silences the compiler warning reported in bug #1411.