wifi: iwlwifi: clean up band in RX metadata

Use u8_get_bits() instead of open-coding, and adjust the
name as well. Also don't use enum nl80211_band for the
variable holding an entirely different type.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250424153620.c41058510800.Ic3f16ce2f0c991fde2cfe7bcd58ee3b875575fce@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>