iwm: Update SCD register accesses

This brings it inline with what's in openbsd. I tested it locally
with 2G and 5G association; it seems to work.

Tested: Intel 7260 AC, hw 0x140, STA mode, 2G/5G

Differential Revision: https://reviews.freebsd.org/D32627
Subscribers: imp
Obtainde from: OpenBSD