iwlagn: simplify error table reading

The current code to read the error table header
just hardcodes all the offsets, which is a bit
hard to understand. We can read in the entire
header (as much as we need) into a structure,
and then take the data from there, which makes
it easier to understand. To read a bigger blob
we also don't need to grab NIC access for each
word read, making the code more efficient.

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