Why is "Hoy" said about Ashur?
Rashi: I made Ashur My staff to rule over My nation.
Radak: He calls out that Ashur should come to Eretz Yisrael to plunder.
Why does it say "Ashur Shevet Api"?
Malbim: It itself does not have authority. Because I was angry at nations, I put a staff in its hand to destroy them.
What is the meaning of "u'Mateh Hu v'Yadam Zami"?
Rashi: Zami (My anger) is a staff in Ashur's hand.
Radak #1: Ashur is Hashem's staff to chastise Yisrael. "V'Yadam" is in their place, i.e. in Eretz Yisrael, I will send this staff.
Radak #2: The staff of My anger is in the hand of Machaneh Ashur.
Malbim: This is connected to verse 7. Ashur is My staff, but Ashur does not realize this. Also, the staff is limited - it is only Zami, via My curse. It cannot strike without this.