wifi: rtlwifi: rtl8192cu: Fix TX aggregation

rtl8192cu is checking rtl_mac.tids when deciding if it should enable
aggregation. This is wrong because rtl_mac.tids is not initialised
anywhere. Check rtl_sta_info.tids instead, which is initialised.

Also, when enabling aggregation also enable RTS. The vendor driver does
this, my router does this. It seems like the thing to do.

Also also, it seems right to set the AMPDU density only when enabling
aggregation.

Also also also, delete the unused member rtl_mac.tids and the unused
macros RTL_AGG_ON and RTL_AGG_OFF.

Naturally, with working AMPDU the download/upload speeds are better.
Before: 59/32 Mbps.
After:  68/46 Mbps.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/4e936334-5f81-403f-a495-0628ebfb6903@gmail.com