ALSA: usb-audio: Simplify rate_min/max and rates set up

There are multiple places in format.c performing the similar code for
setting the rate_min, rate_max and rates fields.  This patch unifies
those in a helper function and calls it at the end of the parser phase
so that all rate_table entries have been already determined.

No functional changes, just a minor code refactoring.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-33-tiwai@suse.de


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