[iwn] fix firmware command use in iwm_auth().

The iwm firmware has separate commands for add, modify and delete for
various things (mac, phy context, etc.) The openbsd driver has a habit
of just completely resetting the NIC each time, which is technically
mostly okay (as long as the reset doesn't actually fail!) but it means
a lot of the code is doing ADD when it should do MODIFY.

The firmware responds in kind - it just asserts.

This fixes auth attempts that occur after the NIC has been already
configured.

(I'm sure there are more instances of this!)

Tested:

iwm0: <Intel Dual Band Wireless AC 7260> mem 0xf1400000-0xf1401fff irq 17 at device 0.0 on pci2
iwm0: revision: 0x140, firmware 25.228 (API ver. 9)

.. STA mode.

Submitted by: Masachika ISHIZUKA <ish@amail.plala.or.jp>