PM: sleep: Update power.smart_suspend under PM spinlock

Put the update of the power.smart_suspend device flag under the PM
spinlock of the device in case multiple bit fields in struct dev_pm_info
occupy one memory location which needs to be updated via RMW every time
any of these bit fields is updated.

The lock in question is already held around the power.direct_complete
flag update in device_prepare() for the same reason, so this change does
not add locking-related overhead to the code.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/2368159.ElGaqSPkdT@rjwysocki.net