mac80211: reduce stack usage in debugfs

We put a few large buffers on the stack here, but it's easy to
just allocate them on the heap, so do that.

Link: https://lore.kernel.org/r/20210920154009.1387f44e7382.Ife043c169e6a44edace516fea9f8311a5ca4282a@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>