iwlwifi: rename IWL_MAX_CMD_TFDS to IWL_MAX_CMD_TBS_PER_TFD

The IWL_MAX_CMD_TFDS name for this constant is wrong, the
constant really indicates how many TBs we can use in the
driver for a single command TFD, rename the constant and
also add a comment explaining it.

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