selftests: devmem: remove unused variable

Trivial fix to unused variable.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250615203511.591438-2-almasrymina@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>