iwlwifi: replace sprintf with scnprintf for debugfs output

The buffersize allocated is not accurate.
Writing to these buffers with scnprintf is safer.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>