iwlwifi: split fw-error-dump between transport and mvm

The mvm op_mode won't allocate the buffer for the transport
any more. The transport allocates its own buffer and mvm
is in charge of splicing the buffers in the debugfs hook.

This makes the repartition easier to handle.

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