iwlwifi: fix 6000 series channel switch command

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.

Cc: stable@kernel.org
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>