docs: devices.rst: fix a C reference markup

The C domain parser of Sphinx3 expects just function names
for :c:func: markups:

	./Documentation/driver-api/pm/devices.rst:413: WARNING: Unparseable C cross-reference: 'device_may_wakeup(dev)'
	Invalid C declaration: Expected end of definition. [error at 17]
	  device_may_wakeup(dev)
	  -----------------^

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