wl1271: Use proper rates for PSM entry/exit null-funcs for 5GHz

A fixed 1 mbps rate was used for the PSM entry/exit null-func frames. Fix this
by using the basic rates instead.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>