greybus: vibrator: convert idr to be an ida

All we need is a simple ida, so use that interface instead of the more
"complex" idr one.  Bonus is we don't need to fix the locking issue I
forgot about when using an idr, as ida has one built-in.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>