fpga: zynq-fpga: Fix unbalanced clock handling

This commit fixes the unbalanced clock handling, where
a failed probe would leave the clock with an enable count of -1.

Reported-by: Josh Cartwright <joshc@ni.com>
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>