iwlwifi: simplify data tracepoint

There's no need to calculate the data_len outside of the tracepoint,
since it's always skb->len - hdr_len, which are both available inside.
Simplify the callers and move the calculation in.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>