[PATCH] libertas: remove adapter->nullpktinterval

No code ever initialized this variable, so it was 0 because of kzalloc().
But no other code changes it, making it rather useless.

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