ALSA: Fix wrong description about hw constraints

The definitions of hw constraint functions are wrongly placed, and the
description about the function is also wrong.
hw_rule_channels_by_format actually refines the channels depending on
the format, not vice versa.

Reported-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>