mac80211: check for allocation failure in debugfs code

kmalloc() can fail.  Also let's move the allocation out of the
declaration block so it's easier to read.

Fixes: 4a5eccaa ("mac80211: Show pending txqlen in debugfs.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>