rtl8723ae: Re-introduce the adaptive rate control

This re-introduces the function rtl8723e_dm_refresh_rate_adaptive_mask.

This function was present in a previous version of the code base,
it works just fine for me -- as long as it is not using stale data.

Unlike the original version of this function it avoids using
dm.undec_sm_pwdb when no beacon was received.

Fixed a style nit in rtl8723e_dm_init_rate_adaptive_mask.

Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>