ALSA: scarlett2: Add missing error checks to *_ctl_get()

The *_ctl_get() functions which call scarlett2_update_*() were not
checking the return value. Fix to check the return value and pass to
the caller.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Fixes: 9e4d5c1b ("ALSA: usb-audio: Scarlett Gen 2 mixer interface")
Link: https://lore.kernel.org/r/32a5fdc83b05fa74e0fcdd672fbf71d75c5f0a6d.1703001053.git.g@b4.vu


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