ALSA: usb-audio: Simplify snd_usb_init_sample_rate() arguments

A preliminary change for the later big changes.  This is a minor code
refactoring to drop the unnecessary arguments that can be retrieved in
a different way.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-20-tiwai@suse.de


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