ALSA: usb-audio: add mono main switch to Presonus S1824c

The 1824c does not have the A/B switch that the 1810c has,
but instead it has a mono main switch that sums the two
main output channels to mono.

Signed-off-by: Roy Vegard Ovesen <roy.vegard.ovesen@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>