wifi: rtw89: mac: define registers of agg_limit and txcnt_limit to share common flow

The agg_limit and txcnt_limit are used by BT-coexistence to reduce
WiFi TX time at once to share time with Bluetooth devices. Since
these registers address are different from WiFi 6 and 7 chips, define
them accordingly.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250217063053.38936-2-pkshih@realtek.com