oeqa: fix return status in pam.py to match shadow-4.2.1

oeqa: fix return status in pam.py to match shadow-4.2.1

The return statuses of commands like `su --help' or `passwd --help'
in shadow 4.2.1 version are different from those in shadow 4.1.4.3
version.

Now that we've upgraded shadow to 4.2.1, we need to fix these statuses
in the pam.py to make things work as expected.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>