[ALSA] use convenient treble scale on WM8750

On Zaurus SL-C3200 (terrier/spitz) based on WM8750, treble scale is
inconveniently reverted (increase level = decrease treble), in opposite
to bass scale, which uses convenient scale.
Fix ALSA WM8750 mixer treble to use convenient treble scale (increase =
increase treble level)
From: Stanislav Brabec <utx@penguin.cz>

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