Fix the following sparse warning:

drivers/firmware/xilinx/zynqmp-debug.c:38:15: warning: symbol
'firmware_debugfs_root' was not declared. Should it be static?
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200415084311.24857-1-yanaijie@huawei.comSigned-off-by: NMichal Simek <michal.simek@xilinx.com>