iwlwifi: add inline helper for packet lengths

Add an inline helper function for getting an RX packet's
length or payload length and use it throughout the code
(most of which I did using an spatch.)

While at it, adjust some code, and remove a bogus comment
from the dvm calibration code.

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