iwlwifi: add debugging for TX path

When debugging TX issues it is helpful to know the seq nr of the
frame being transmitted. The seq nr is printed as part of ucode's
log informing us which frame is being processed. Having this information
printed in driver log makes it easy to match activities between driver
and firmware.

Also make possible to print TX flags directly. These are already printed
as part of entire TX command, but having it printed directly in cpu format
makes it easier to look at.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>