wifi: rtw89: rename roc_entity_idx to roc_chanctx_idx

The target enum has been renamed to rtw89_"chanctx"_idx. So for
readability, rename roc_entity_idx to roc_"chanctx"_idx to align.

No logic is changed.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240819091724.33730-8-pkshih@realtek.com