iwlwifi: mvm: don't store section offset/length in debugfs

When different images can (soon) be accessed through this
file, storing the section offset/length on first access to
the file breaks (or needs manual reset). Avoid this by not
storing the offset/length values but using them locally in
the function only. That way, the correct values are always
used.

While at it, correct the check that firmware is loaded.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>