libertas: before sleeping, check for a command result

If we don't check for a command response early, but rather sleep,
then we might sleep despite an already-received command response.
This will lead to a command-timeout.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>