usecase: resolve PID file path issue in L2 switching usecase

The PID file paths in 'stop.sh' are not consistent with the
other scripts and need to be corrected.

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: Ic4d68d2da5e9f8274094de505a0c305263818539