ALSA: emu10k1: Fix annotation and cast for the recent uapi header change

The recent sound/emu10k1.h uapi header change by the commit
2e468867 ("ALSA: emu10k1: Make uapi/emu10k1.h compilable again")
made sparse angry because of the inconsistency of __user annotation
and the own ctl id struct that were changed in uapi header.

This patch addresses those by adjusting the cast and annotations
properly again.

Fixes: 2e468867 ("ALSA: emu10k1: Make uapi/emu10k1.h compilable again")
Link: https://lore.kernel.org/r/20200202090724.18232-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>