fpga: fpga-mgr: wrap the state() op

An FPGA manager should not be required to provide a state() op.
Add a wrapper consistent with the other op wrappers.
Move op check to wrapper.
Default to FPGA_MGR_STATE_UNKNOWN, what noop state() ops use.
Remove unneeded noop state() ops

[mdf@kernel.org: Reworded first line]
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>