docs: mei.rst: fix a C expression markup

Sphinx 3.x doesn't allow expressions using :c:func markup:

	./Documentation/driver-api/mei/mei.rst:41: WARNING: Unparseable C cross-reference: 'close(int fd)'
	Invalid C declaration: Expected end of definition. [error at 5]
	  close(int fd)
	  -----^

So, convert it into a :c:expr.

Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>