iwlagn: support multiple TBs per command

iwlagn: support multiple TBs per command

The current "huge" command handling is a bit
confusing, and very limited since only one
command may be huge at a time. Additionally,
we often copy data around quite pointlessly
since we could instead map the existing scan
buffer for example and use it directly.

This patch makes that possible. The first
change is that multiple buffers may be given
to each command (this change was prepared
earlier so callsites don't need to change).
Each of those can be mapped attached to a TB
in the TFD, and the command header can use a
TB (the first one) in the TFD as well.

Doing this allows getting rid of huge commands
in favour of mapping existing buffers. The
beacon transmission is also optimised to not
copy the SKB at all but use multiple TBs.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>