rtw88: Use RF_CFGCH instead of hard coded 0x18

While debugging some funky register reads of 0xaeaea from RF_CFGCH
resulting in "rtw880: [TXGAPK] unknown channel 234!!" more of these
reads came to my attention hidden by using the register index rather
than the defined value. Make this more grep-able.

MFC after: 3 days