ath6kl: Fix endpoint_stats debugfs buffer length calculation

The previous version did not really make much sense and the theoretical
maximum length would be a bit longer. Calculate the length more
accurately. In addition, there is no need to clear the buffer, so use
kmalloc instead of kzalloc. For bonus points, add the forgotten
cred_rpt_from_other value to the file.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>