[iwm] Move Smart Fifo handling into if_iwm_sf.c, sync with Linux iwlwifi.

• This change also fixes a possible issue in the existing smart-fifo code, which set the IWM_SF_CFG_DUMMY_NOTIF_OFF bit on AC8260 chipsets, although that's only used in iwlwifi for Family 8000 chipsets connected via SDIO interface.

Obtained from: Dragonflybsd.git cb650b01526b0aeef3c4307d926e7f1428997d50