asix-sigma: more trigger LUT download rephrase, think 16bit entities

asix-sigma: more trigger LUT download rephrase, think 16bit entities

Further rephrase the sigma_write_trigger_lut() routine. It's helpful to
"think" in BE16 quantities to improve readability of LUT address and
parameter downloads. Better matches the vendor's documentation. Also use
a better name for the "trigger select 2" register content.