libertas: fix command timeout after firmware failure

This is a fix for OLPC ticket #6586: "SCAN command fails, timer doesn't
fire". In fact, the timer was firing; the problem was that the dnld_sent
state variable was not being updated after the timer expired, so
lbs_execute_next_command was not being called.

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>