coresight: tgu: fix array access violation

The maximum value of grp_refcnt is greater than the upper bound of
the array grp_data, so accessing the array through it will cause
array out-of-bounds problem.

Change-Id: I8311d5f5916aa4fb05f235986acc494156c39937
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>