wcn36xx: Fix reported rx_highest rate

Qualcomm's document "80-WL007-1 Rev. J" states that the highest rx rate for
the WCN3660 and WCN3680 on MCS 0-7 is 150 Mbps not the 72 stated here.
Given only the WCN3660 and WCN3680 can connect to 5Ghz frequencies the
value specified here is clearly incorrect.

72 Mbps is what the downstream Prima incorrectly reports for this value and
that spurious value appears to have been replicated into upstream.

Fixes: 8e84c258 ("wcn36xx: mac80211 driver for Qualcomm WCN3660/WCN3680
hardware")

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>