include/uapi/sound/firewire.h: use "_UAPI" instead of "UAPI"

When installing, "scripts/headers_install.sh" will strip guard macro'
"_UAPI" to prevent from appearing it to users. And also, all another
files which need uapi prefix always use "_UAPI", not "UAPI".

So use "_UAPI" instead of "UAPI" on the guard macro, and also give a
comment for "#endif".

Signed-off-by: Chen Gang <gang.chen@asianux.com>