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

An FPGA manager should not be required to provide a
write_sg function. Move the op check to the wrapper.
Default to -EOPNOTSUP so its users will fail
gracefully.

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