staging: wilc1000: fixed kernel panic when firmware is not started

This patch fixed the problems caused by if firmware is not started.
That is why, in nl80211 put current TX power in interface info.
If firmware is not started, this function(get_tx_power) does not work.

Signed-off-by: Leo Kim <leo.kim@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>