iwlegacy: fix IBSS mode crashes

We should not switch to non-IBSS channels when working in IBSS mode,
otherwise there are microcode errors, and after some time system
crashes.

This bug is only observable when software scan is used in IBSS mode,
so should be considered as regression after:

commit 0263aa45
Author: Stanislaw Gruszka <sgruszka@redhat.com>
Date:   Tue Mar 29 11:24:21 2011 +0200

    iwl3945: disable hw scan by default

However IBSS mode check, which this patch add again, was removed by

commit b2f30e8b


Author: Johannes Berg <johannes.berg@intel.com>
Date:   Thu Jan 21 07:32:20 2010 -0800

    iwlwifi: remove IBSS channel sanity check

That commit claim that mac80211 will not use non-IBSS channel in IBSS
mode, what definitely is not true. Bug probably should be fixed in
mac80211, but that will require more work, so better to apply that patch
temporally, and provide proper mac80211 fix latter.

Resolves:
https://bugzilla.kernel.org/show_bug.cgi?id=34452

Reported-and-tested-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: stable@kernel.org # 2.6.38.5+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>