TXRX_CSR5 stores allowable ACK rates instead of basic rates, so rename

TXRX_CSR5 stores allowable ACK rates instead of basic rates, so rename
rt2661_set_basicrates() to rt2661_set_ackrates().  Also turn on 6Mbits/s,
12Mbits/s and 24Mbits/s in ACK rates, if we are not forced to be in 11b
mode and to start/join an ERP BSS.  Now rt2661 part will send ACK at
24Mbits/s instead of 6Mbits/s, if the data to be acknowledged are sent at
rate above/equal 24Mbits/s, which is standard conforming and improves
performance a bit.

# This problem is identified by running following command on any NIC
# that can capture control frames and is aware of ERP:
#   tcpdump -ni iface -y ieee802_11_radio not wlan type mgt
# Pay attention to the rate, at which ACK gets sent ;]