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

sys/kern/genassym.sh

@@ -57,7 +57,7 @@ do
 	*)	usage;;
 	esac
 done
-shift $(($OPTIND - 1))
+shift $((OPTIND - 1))
 case $# in
 1)	;;
 *)	usage;;
@@ -65,8 +65,8 @@ esac
 
 if [ "$use_outfile" = "yes" ]
 then
-	work $1  3>"$outfile" >&3 3>&-
+	work "$1"  3>"$outfile" >&3 3>&-
 else
-	work $1
+	work "$1"
 fi