ALSA: hda/realtek - Simplify Dell XPS13 GPIO handling

Dell XPS13 has multi-step fixups, and one of them
(ALC288_FIXUP_DELL_XPS_13_GPIO6) corresponds to the management of GPIO
bit6 (0x40).  It used to be a static init verbs (to turn *off* the
bit6).

In this patch, we convert it as the gpio_mask and gpio_dir
initializations folded in the existing fixup function.  With this
change, ALC288_FIXUP_DELL_XPS_13_GPIO6 becomes superfluous, thus it's
removed.

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