tools: cpu-hotplug fix unexpected operator error

on-off-test is a bash script and invoked from /bin/sh
This results in the following error:

./on-off-test.sh: 9: [: !=: unexpected operator

Changed Makefile to use bash instead.

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>