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

An FPGA manager is not required to provide a fpga_remove() op.
Add a wrapper consistent with the other op wrappers.
Move op check to wrapper.

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