ALSA: x86: Make CONFIG_SND_X86 bool

CONFIG_SND_X86 is a menu config to filter only for x86-specific
drivers in its sub-menu, and this doesn't have to be tristate but
rather it should be a bool.  Also, like other sub-menu configs, it's
more user-friendly to be default=y; it's merely a menu config and the
actual drivers are configured in the sub-menu, after all.

Fixes: 287599cf ("ALSA: add Intel HDMI LPE audio driver for BYT/CHT-T")
Signed-off-by: Takashi Iwai <tiwai@suse.de>