iwlwifi: mvm: use dev_coredumpsg()

iwlmvm currently uses dev_coredumpm() to collect multiple
buffers, but this has the downside of pinning the module
until the coredump expires, if the data isn't read by any
userspace.

Avoid this by using the new dev_coredumpsg() method. We
still copy the data from the old way of generating it, but
neither hold on to vmalloc'ed data for a long time, nor do
we pin the module now.

Signed-off-by: Aviya Erenfeld <aviya.erenfeld@intel.com>
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>