fpga: zynq-fpga: Fix issue with drvdata being overwritten.

Upon registering a FPGA Manager low level driver, FPGA Manager
core overwrites the platform drvdata pointer. Prior to this commit
zynq-fpga falsely relied on this pointer to still be valid at remove()
time.

Reported-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Acked-by: Alan Tull <atull@opensource.altera.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>