fpga: fpga-mgr: remove unnecessary code in __fpga_mgr_get

Notice that mgr = to_fpga_manager(dev); expands to:

mgr = container_of(dev, struct fpga_manager, dev);

and container_of is never null, so this null check is
unnecessary.

Addresses-Coverity-ID: 1397916
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>