ALSA: oxygen: Fix logical-not-parentheses warning

This fixes the following warning, that is seen with gcc 5.1:
warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses].

Signed-off-by: Tomer Barletz <barletz@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>