ALSA: 6fire: Fix input volume change detection

ALSA: 6fire: Fix input volume change detection

usb6fire_control_input_vol_put() stores the analog capture volume
as a signed offset in rt->input_vol[] (-15..+15), but it compares
the cached value against the user-visible mixer value (0..30)
before subtracting 15.

This mixes two domains in the change detection path. Since the
runtime is zero-initialized, the visible default is 15; writing 0
right after probe is ignored, while writing 15 is reported as a
change even though the cached value remains 0.

Normalize the user value before comparing it with the cached offset.

Fixes: 06bb4e743501 ("ALSA: snd-usb-6fire: add analog input volume control")
Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260416-alsa-6fire-input-volume-change-detection-v1-1-ec78299168df@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>