Using redirection with sudo should take some care. Your command line executes the echo command with root privileges then
tries to store the output in /etc/modules as regular user. From the sudo man page there is an useful example:
To make a usage listing of the directories in the /home partition. Note that this runs the commands
in a sub-shell to make the cd and file redirection work.
$ sudo sh -c "cd /home ; du -s * │ sort -rn > USAGE"
So you have to spawn the process in a subshell, using
sudo sh -c "echo ath_pci >> /etc/modules"