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

An FPGA manager should not be required to provide a
write_init() op if there is nothing for it do.
So add a wrapper and move the op checking.
Default to success.

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