docs: fpga: replace :c:member: macros

Those macros are not doing the right thing with Sphinx 3,
causing parse errors:

	./Documentation/driver-api/fpga/fpga-mgr.rst:104: WARNING: Unparseable C cross-reference: 'fpga_manager->state'
	Invalid C declaration: Expected end of definition. [error at 12]
	  fpga_manager->state
	  ------------^
	./Documentation/driver-api/fpga/fpga-programming.rst:18: WARNING: Unparseable C cross-reference: 'fpga_region->info'
	Invalid C declaration: Expected end of definition. [error at 11]
	  fpga_region->info
	  -----------^
	./Documentation/driver-api/fpga/fpga-region.rst:62: WARNING: Unparseable C cross-reference: 'fpga_region->bridge_list'
	Invalid C declaration: Expected end of definition. [error at 11]
	  fpga_region->bridge_list
	  -----------^
	./Documentation/driver-api/fpga/fpga-region.rst:62: WARNING: Unparseable C cross-reference: 'fpga_region->get_bridges'
	Invalid C declaration: Expected end of definition. [error at 11]
	  fpga_region->get_bridges
	  -----------^

Replace them by :c:expr:, with does what's desired.

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