selftests/ftrace: Fix checkbashisms errors

Fix a test case to make checkbashisms clean.

Signed-off-by: Masami Hiramatsu <[e-mail]>
Acked-by: Steven Rostedt (VMware) <[e-mail]>
Signed-off-by: Shuah Khan (Samsung OSG) <[e-mail]>

tools/testing/selftests/ftrace/test.d/trigger/trigger-trace-marker-snapshot.tc

···
 	fi
 	echo "testing $line for >$x<"
 	match=`echo $line | sed -e "s/>$x<//"`
-	if [ "$line" == "$match" ]; then
+	if [ "$line" = "$match" ]; then
 	    fail "$line does not have >$x< in it"
 	fi
-	let x=$x+2
+	x=$((x+2))
     done
 }