greybus: svc: free pwrmon_rails memory upon exit

For every time SVC instance is created, memories for storing the rail IDs
are allocated, however, they are not freed when the SVC is destroyed.
This patch fixes the memory leak by freeing the memory when debugfs for
SVC is no longer needed.

Testing Done:
- Check pwrmon debugfs after turning on and off SVC

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>