iwlegacy: dump stack when fail to gain access to the device

Print dump stack when the device is not responding. This should give
some more clue about the reason of failure. Also change the message we
print, since "MAC in deep sleep" is kinda confusing.

On the way add unlikely(), as fail to gain NIC access is hmm ...
unlikely.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>