mfd: exynos-lpass: Mark PM functions as __maybe_unused

The newly added exynos lpass driver produces a build warning when
CONFIG_PM is disabled since the only callers of exynos_lpass_disable
are under an #ifdef:

drivers/mfd/exynos-lpass.c:93:13: error: 'exynos_lpass_disable' defined but not used [-Werror=unused-function]
 static void exynos_lpass_disable(struct exynos_lpass *lpass)

This removes the #ifdef and replaces it with __maybe_unused annotations
so the compiler can leave out the unused code silently with less
room for mistakes.

Fixes: 36c26760bba8 ("mfd: Add Samsung Exynos Low Power Audio Subsystem driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>