iwlwifi: mvm: port to devcoredump framework

iwlwifi features a debug mechanism that allows to dump
binary data which is helpful to debug the firmware.
Until now, this data was made available for the userspace
through debugfs. For this exact purpose, devcoredump was
created. Move to the new infrastructure.

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