iwlwifi: fix 6000 series channel switch command

commit 8f7b8db6e0557c8437adf9371e020cd89a7e85dc upstream.

The channel switch command for 6000 series devices
is larger than the maximum inline command size of
320 bytes. The command is therefore refused with a
warning. Fix this by allocating the command and
using the NOCOPY mechanism.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>