selftests/ftrace: Fix checkbashisms errors

Fix the below checkbashisms errors. Because of these errors, these tests
will fail on dash shell.

possible bashism in test.d/kprobe/kretprobe_entry_arg.tc line 14 ('function' is useless):
function streq() {
possible bashism in test.d/dynevent/fprobe_entry_arg.tc line 14 ('function' is useless):
function streq() {

Fixes: f6e2253a

 ("selftests/ftrace: Add test cases for entry args at function exit")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>