fpga manager: remove unnecessary null pointer checks

Remove unnecessary null pointer checks.  We want the caller of
these functions to do their own pointer checks.  Add some
comments to document this.

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