Revert "ALSA: usb-audio: Disable Lenovo P620 Rear line-in volume control"

Revert "ALSA: usb-audio: Disable Lenovo P620 Rear line-in volume control"

This reverts commit 34dedd2a83b241ba6aeb290260313c65dc58660e.

According to Realtek, volume FU works for line-in.

I can confirm volume control works after device firmware is updated.

Fixes: 34dedd2a83b2 ("ALSA: usb-audio: Disable Lenovo P620 Rear line-in volume control")
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200915103925.12777-1-kai.heng.feng@canonical.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>