wifi: cfg80211: debugfs: fix return type in ht40allow_map_read()

The return type is supposed to be ssize_t, which is signed long,
but "r" was declared as unsigned int.  This means that on 64 bit systems
we return positive values instead of negative error codes.

Fixes: 80a3511d ("cfg80211: add debugfs HT40 allow map")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YutvOQeJm0UjLhwU@kili


Signed-off-by: Johannes Berg <johannes.berg@intel.com>