Revert "iwlwifi: remove wide_cmd_header field"

This reverts commit 0a8159cb.

It turns out that this flag is used by iwldvm, so we can't get rid of
it.  This broke iwldvm devices with BAD_COMMAND errors.

Fixes: 0a8159cb ("iwlwifi: remove wide_cmd_header field")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201009122123.3e4ee0ad7a71.Id6d95ae601f048aeb4d2ed63a1712e469da84369@changeid