[otbn,rtl] Fix `adder_y_res_used`
[otbn,rtl] Fix `adder_y_res_used`

`adder_y_res_used` indicates whether `adder_y_res` is used in any way.
This is relevant for determining if upstream integrity errors could
possibly have any effect.  Prior to this commit, `adder_y_res_used` was
`0` in one case where `adder_y_res` is used in an `if` condition.  This
is wrong, and this commit fixes this flaw.

Signed-off-by: Andreas Kurth <62c4a7f75976b7a2fb8d79afddaccc3e9236aad5@lowrisc.org>

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