ALSA: pcm: Allow dropping stream directly after resume

So far, the PCM core refuses DROP ioctl when the stream in the
suspended state.  This was basically to avoid the invalid state change
*during* the suspend.  But since we protect the power change globally
in the common PCM ioctl caller side, it's guaranteed that
snd_pcm_drop() is called at the right power state.  So we can assume
that the drop of stream is safe immediately after SUSPENDED state.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>