[iwm] if_iwm - Factor out and improve iwm_mvm_scan_rxon_flags() in if_iwm_scan.c.

From the original commit:

• Actually look at the first channel in the list. If it's a 2.4GHz channel, set IWM_PHY_BAND_24 flag. The IWM_PHY_BAND_5 flag is 0 anyway, so we don't need to look further.

• While there factor out the iwm_mvm_rrm_scan_needed() tlv capability check.

Taken-From: Linux iwlwifi

However, this only really does the latter. The sc_ic channel list isn't the
scan channel list, it's the /whole list/ for the set of active channels,
so I don't know what the right thing to do is here.

So I'll commit this as an intermediary commit and we'll have to revisit whether
to finish the refactor as-is.

Tested:

• Intel 7260, STA mode

Obtained from: dragonflybsd.git 53a009d6f66108b40d622ed90ea95eba5c0e5432