libertas: submit RSSI command on tx timeout, to check whether module is dead

We don't necessarily want to reset the device on a TX timeout. But more
often than not, the real cause is that the firmware has crapped itself,
not just that the network is busy. So submit any harmless command, and
if _that_ times out, then the error handling code will reset the module,
as appropriate.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>