wl1271: fix ps scheme in wl1271_op_conf_tx()

Commit "wl1271: implement WMM" accidentally used CONF_PS_SCHEME_LEGACY_PSPOLL,
but instead CONF_PS_SCHEME_LEGACY should be used as earlier.

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