docs: get rid of :c:type explicit declarations for structs

The :c:type:`foo` only works properly with structs before
Sphinx 3.x.

On Sphinx 3.x, structs should now be declared using the
.. c:struct, and referenced via :c:struct tag.

As we now have the automarkup.py macro, that automatically
convert:
	struct foo

into cross-references, let's get rid of that, solving
several warnings when building docs with Sphinx 3.x.

Reviewed-by: André Almeida <andrealmeid@collabora.com> # blk-mq.rst
Reviewed-by: Takashi Iwai <tiwai@suse.de> # sound
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/fpga/fpga-mgr.rst

@@ -102,8 +102,8 @@ API for implementing a new FPGA Manager driver
 ----------------------------------------------
 
 * ``fpga_mgr_states`` —  Values for :c:member:`fpga_manager->state`.
-* struct :c:type:`fpga_manager` —  the FPGA manager struct
-* struct :c:type:`fpga_manager_ops` —  Low level FPGA manager driver ops
+* struct fpga_manager —  the FPGA manager struct
+* struct fpga_manager_ops —  Low level FPGA manager driver ops
 * devm_fpga_mgr_create() —  Allocate and init a manager struct
 * fpga_mgr_register() —  Register an FPGA manager
 * fpga_mgr_unregister() —  Unregister an FPGA manager