um: vector: fix bpfflash parameter evaluation

With W=1 the build complains about a pointer compared to
zero, clearly the result should've been compared.

Fixes: 9807019a ("um: Loadable BPF "Firmware" for vector drivers")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>