fpga: fpga-mgr: make write_complete() op optional

An FPGA manager should not be required to provide a
write_complete function if there is nothing.  Move
the op check to the existing wrapper.
Default to success and remove noop function.

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