ALSA: scarlett2: Minor refactor MSD mode check

Create local variable for storing private data pointer in
snd_scarlett2_controls_create(). It's currently only used for checking
msd_switch, but it will be used again.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/ab50939dca0fdc5fa3493fc8eee3a2fdefffa62d.1703612638.git.g@b4.vu