libertas: fix a potential NULL pointer dereference

commit 7da413a18583baaf35dd4a8eb414fa410367d7f2 upstream.

alloc_workqueue is not checked for errors and as a result,
a potential NULL dereference could occur.

Signed-off-by: Allen Pais <allen.pais@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
[krzk: backport applied to different path - without marvell subdir]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>