docs: libata.rst: fix a wrong usage of :c:type: tag

The usage of :c:type: to reference to a struct member is wrong,
as pointed by Sphinx 3:

	./Documentation/driver-api/libata.rst:511: WARNING: Unparseable C cross-reference: 'qc->complete_fn'
	Invalid C declaration: Expected end of definition. [error at 2]
	  qc->complete_fn
	  --^

Instead, let's use :c:expr: for such purpose.

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