[iwm] Move struct iwm_cfg chipset configs to if_iwm_7000.c and _8000.c

• This is more similar to how code/definitions are distributed in Linux's iwlwifi.

• This should make recognizing new chipset variants, and adding additional flags from the Linux iwlwifi code easier, without blowing up if_iwm.c

Obtained from: dragonflybsd.git 27d11320e707d2c41424efc1983762f6799941d6