ALSA: usb-audio: scarlett2: Merge common line in capture strings

Use a common sprintf() format for the mixer element names generated in
scarlett2_add_line_in_ctls() in preparation for more of them.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/d91dce45ac75a541c21f47540ecbda24bd83f68c.1624294591.git.g@b4.vu


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