fpga: stratix10-soc: Fix return value check in s10_ops_write_init()

fpga: stratix10-soc: Fix return value check in s10_ops_write_init()

In case of error, the function stratix10_svc_allocate_memory()
returns ERR_PTR() and never returns NULL. The NULL test in the
return value check should be replaced with IS_ERR().

Fixes: e7eef1d7633a ("fpga: add intel stratix10 soc fpga manager driver")
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Russ Weight <russell.h.weight@intel.com>
Cc: stable@vger.kernel.org
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20221126071430.19540-1-zhengyongjun3@huawei.com
Signed-off-by: Xu Yilun <yilun.xu@intel.com>