fpga: altera-freeze-bridge: Address warning about unused variable

warning: unused variable 'altera_freeze_br_of_match'
 [-Wunused-const-variable]
  static const struct of_device_id altera_freeze_br_of_match[] = {

Fixes: ca24a648 ("fpga: add altera freeze bridge support")
Cc: Tom Rix <trix@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>