Use the new pm_sleep_ptr() macro to handle the .irq_set_wake() callback.

This macro allows the mt6397_irq_set_wake() function to be automatically
dropped by the compiler when CONFIG_SUSPEND is disabled, without having
to use #ifdef guards.

This has the advantage of always compiling these functions in,
independently of any Kconfig option. Thanks to that, bugs and other
regressions are subsequently easier to catch.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Lee Jones <lee@kernel.org>