ALSA: control: Consolidate helpers for adding and replacing ctl elements

Both snd_ctl_add() and snd_ctl_replace() process the things in a
fairly similar way, and indeed the most of the codes can be unified.

This patch is a refactoring to consolidate the both functions to call
a single helper with an extra "mode" argument.  There should be no
functional difference, except for one additional sanity check applied
now to snd_ctl_replace() (which was rather overlooking, IMO), too.

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