[ALSA] fix comments in sound/core.h

Two sentences seem to be spliced into one in comment, fix that and fix
english. Also fix codingstyle.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>