libertas: clean up is_command_allowed_in_ps()

Total overkill to have an array when there's only one command in it.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>