libertas: check whether bus can do more than polling

If a sdio host does not support sdio irqs, polling is used
instead. That has an impact on performance. Some functionality
should not be enabled then. This add a variable in
libertas_priv to indicate that.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>