wl1271: Use all basic rates for ps-poll, instead of just the slowest

This improves ps-poll handling in WLAN PSM. It also improves performance,
including WLAN-BT co-existence reliability.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>