ALSA: rawmidi: Minor code refactoring

Unify a few open codes with helper functions to improve the
readability.  Minor behavior changes (rather fixes) are:
- runtime->drain clearance is done within lock
- active_sensing is updated before resizing buffer in
  SNDRV_RAWMIDI_IOCTL_PARAMS ioctl.
Other than that, simply code cleanups.

Signed-off-by: Takashi Iwai <tiwai@suse.de>