ALSA: usb-audio: scarlett2: Add scarlett2_vol_ctl_write() helper

Add helper function for setting the read/write status of a volume
control. This will simplify the upcoming mute control support.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/00dd57e1abb3fa379fb51d4ac8537dbddc09f0ea.1624294591.git.g@b4.vu


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