ALSA: scarlett2: Remove line_out_hw_vol device info entry

By splitting config set gen2 into gen2a/b (for 6i6/18i8 vs 18i20), and
gen3b into gen3b/c (for 4i4/8i6 vs 18i8/18i20), we can use
scarlett2_has_config_item() instead of the per-device line_out_hw_vol.

As Gen 4 has a master volume control but no SW/HW switches, check for
both SCARLETT2_CONFIG_MASTER_VOLUME and SCARLETT2_CONFIG_SW_HW_SWITCH
as needed, even though for Gen 2 and Gen 3 the former implies the
latter.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/307c4f8d6d2e034f3e386b51d72a39d77c8a9fce.1703444932.git.g@b4.vu


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