fpga: of-fpga-region: Use platform_set_drvdata

Use platform_set_drvdata rather than dev_set_drvdata
to match the platform_get_drvdata in the _remove()
function of the platform driver.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>