iwlwifi: fix length check in multi-TB HCMD

As reported by Ben Hutchings, there was a harmless issue in
the checks being done on the lengths of the TBs while
building the TFD for a multi-TB host command.

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