fpga: Fix dead store in fpga-bridge.c

Using clang's scan-build/view this issue was flagged
a dead store issue in fpga-bridge.c

warning: Value stored to 'ret' is never read [deadcode.DeadStores]
                  ret = id;

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