ALSA: scarlett2: Add PCM Input Switch for Solo Gen 4

When the Direct button on the Solo Gen 4 is held for 3 seconds, the
PCM 1 and 2 inputs are toggled between DSP Outputs 1 and 2, and Mixer
Outputs E and F. This patch adds the corresponding ALSA control.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/8c67c6131c459588ac4edab11e1fbc40a8297328.1703612638.git.g@b4.vu