ALSA: usb-audio: scarlett2: Add usb_tx/rx functions

Pull out snd_usb_ctl_msg() calls from scarlett2_usb() and put into
scarlett2_usb_tx() and scarlett2_usb_rx() functions.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/720d1d6f227fc8f5d7d6191a8de62db796940235.1624294591.git.g@b4.vu


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