libertas: remove some pointless checks for cmdnode buffer being present

We allocate them all at the same time, at startup. If they go missing,
we have more serious things to worry about, and the resulting oops will
be a perfectly acceptable result.

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