PM / devfreq: remove a duplicated kernel-doc markup

The update_devfreq() is also documented at devfreq.c, which
has a more complete note.

So, drop the duplicated markup, in order to avoid this
warning:
	.../Documentation/driver-api/device_link.rst: WARNING: Duplicate C declaration, also defined in 'driver-api/infrastructure'.
	Declaration is 'device_link_state'.

(and to cause a problem with cross-references to it)

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>