wl1271: exit ELP mode when setting enabled rates in tx

This bug was being triggered by a call to acx_rate_policies in tx_work
without calling ps_elp_wakeup first.  If we have full PSM enabled, this
happens rather often, immediately after association.

Reported-by: Tuomas Katila <ext-tuomas.2.katila@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Tested-by: Tuomas Katila <ext-tuomas.2.katila@nokia.com>