ath6kl: use a larger buffer for debug output

The return value of snprintf() is the number of bytes which would
have been copied if there was enough space, but we want the number of
bytes actually copied.  The scnprintf() function does this.

Also in theory, a %u can take take 10 digits so we may as well make
the buffer larger as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>