perf tests stat_bpf_counters: Fix usage of '==' to address shellcheck warning

perf tests stat_bpf_counters: Fix usage of '==' to address shellcheck warning

Running shellcheck on stat_bpf_counter.sh generates below
warning:

   In tests/shell/stat_bpf_counters.sh line 34:
   if [ "$base_cycles" == "<not" ]; then
                       ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.

   In tests/shell/stat_bpf_counters.sh line 39:
   if [ "$bpf_cycles" == "<not" ]; then
                      ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.

Fix this by using "=" instead of "==" to work for
all shells.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-10-atrajeev@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>