ALSA: usb-audio: Handle discrete rates properly in hw constraints

In the current code, when the device provides the discrete sample rate
tables with unusual sample rates, the driver tries to gather the whole
values from the audioformat entries and create a hw-constraint rule to
restrict with this single rate list.  This is rather inefficient and
may overlook the rates that are associated only with the certain
audioformat entries.

This patch improves the hw constraint setup by rewriting the existing
hw_rule_rate().  The discrete sample rates (identified by rate_table
and nr_rates of format entry) are checked in the existing
hw_rule_rate() instead of extra rules; in the case of discrete rates,
the function compares with each rate table entry and calculates the
min/max values from there.  For the contiguous rates, the behavior
doesn't change.

Along with it, snd_usb_pcm_check_knot() and snb_usb_substream
rate_list field become superfluous, thus those are dropped.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-2-tiwai@suse.de


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