fpga: region: fix kernel-doc

Fix function name in of-fpga-region.c kernel-doc comment
to remove a warning found by running scripts/kernel-doc,
which is caused by using 'make W=1'.

drivers/fpga/of-fpga-region.c:451: warning: expecting prototype for
fpga_region_init(). Prototype was for of_fpga_region_init() instead.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>