FogBugz #536771: fpga: bridge: fix klocwork sprintf complaint

commit 3a3cbaa8a73de6cf40ac26e8028f6789c745bb2e from
https://github.com/altera-opensource/linux-socfpga.git

 socfpga-6.6.22-lts.

Klocwork give the following complaints:

/home/yves/linux/intel/linux-socfpga/drivers/fpga/fpga-bridge.c:243 --
-- SV.BANNED.REQUIRED.SPRINTF (4:Review) Analyze Function 'sprintf' is
deprecated.  Replace with more secure equivalent like 'sprintf_s', add
missing logic, or re-architect.

/home/yves/linux/intel/linux-socfpga/drivers/fpga/fpga-bridge.c:255 --
-- SV.BANNED.REQUIRED.SPRINTF (4:Review) Analyze Function 'sprintf' is
deprecated.  Replace with more secure equivalent like 'sprintf_s', add
missing logic, or re-architect.

Remedy these by using scnprintf rather than sprintf.

Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Liwei Song <liwei.song@windriver.com>