rtw88: use #define for NL80211_BAND_2GHZ instead of hardcoded number

Use NL80211_BAND_2GHZ instead of a hard coded 0 as array index for the
band. While LinuxKPI provides a KPI compatibility some of these values
may not necessarily be KBI compatible (in this case they shoule be so
this is a NOP) and after all it is better style.

No functional change.

MFC after: 3 days