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>

Documentation/driver-api/libata.rst

@@ -508,7 +508,7 @@ also complete commands.
 
 2. ATA_QCFLAG_ACTIVE is cleared from qc->flags.
 
-3. :c:func:`qc->complete_fn` callback is invoked. If the return value of the
+3. :c:expr:`qc->complete_fn` callback is invoked. If the return value of the
    callback is not zero. Completion is short circuited and
    :c:func:`ata_qc_complete` returns.