[PATCH] libertas: remove adapter->listeninterval

listeninterval was initialized with MRVDRV_DEFAULT_LISTEN_INTERVAL, but
there exists that would ever change it. So we can use this define directly.

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