isa/bcd.mdwn: update addg6s pseudocode

isa/bcd.mdwn: update addg6s pseudocode

Not only this pseudocode is wrong, since some actions are put inside
the loop, but also the algorithm as is cannot be translated due to
use of function call (carry_out). For more details, check these
discussions:

	https://libre-soc.org/irclog/%23libre-soc.2021-07-28.log.html
	https://bugs.libre-soc.org/show_bug.cgi?id=656