fpga: zynqmp-fpga: fix compile warning in fpga_mgr_register

This fixes the compile warning as below:

drivers/fpga/fpga-mgr.c:911:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
911 | struct dentry *d, *parent;
		^~~~~~

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>