thermal: trip: Split thermal_zone_device_set_mode()

[ Upstream commit e5f98896efb3b6350cb6f1c241394966dcbcf240 ]

Pull a wrapper around thermal zone .change_mode() callback out of
thermal_zone_device_set_mode() because it will be used elsewhere
subsequently.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2206793.irdbgypaU6@rjwysocki.net
Stable-dep-of: f7c1b0e4ae47 ("thermal: core: Back off when polling thermal zones on errors")
Signed-off-by: Sasha Levin <sashal@kernel.org>