ALSA: usb-audio: Fix Focusrite Scarlett 6i6 gen1 - input handling

The Scarlett 6i6 has no padding on rear inputs 3/4 but a gainstage.
This patch introduces this functionality as to be seen in the mac
or windows scarlett control.

The correct address could already be found in the dump info, but was
never used. Without this patch inputs 3/4 are quite unusable else.

Signed-off-by: Jens Verwiebe <info@jensverwiebe.de>
Link: https://lore.kernel.org/r/384d65cd-5e87-91eb-9fc3-e57226f534c6@jensverwiebe.de


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