genassym.sh: Fix two minor issues found by shellcheck

o Remove redunant $ in $(( )) expression.
o Quote arg passed to work so paths with spaces, etc will work.

MFC After: 2 weeks
Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31335

(cherry picked from commit 46dd3ef0338ba2ad24d05bab2a614410cf17b017)