iwlwifi: mvm: fix tx seq_ctrl debug print

Since seq_number is incremented right after using
it, so printed seq_ctrl was actually the next
one to be used.

Fix it by incrementing the seq_number only later,
before saving it.

Additionally, use the IEEE80211_SEQ_TO_SN macro
in order to print the actual sequence number.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>