fpga: prevent integer overflow in dfl_feature_ioctl_set_irq()

The "hdr.count * sizeof(s32)" multiplication can overflow on 32 bit
systems leading to memory corruption.  Use array_size() to fix that.

Fixes: 322b598b ("fpga: dfl: introduce interrupt trigger setting API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/YxBAtYCM38dM7yzI@kili


Signed-off-by: Xu Yilun <yilun.xu@intel.com>