FMC: show_sdb_tree: fix offset calculation

The code reported wrong addresses in the sdb dumps.  All sdb addresses
are relative, but the code was adding the base address twice.  Bug
exposed by a gateware image with two bridge levels.

Thanks David for reporting the problem.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Reported-by: Juan David Gonzalez Cobas <dcobas@cern.ch>
Tested-by: Juan David Gonzalez Cobas <dcobas@cern.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>