net: mctp: tests: use actual address when creating dev with addr

Sashiko reports:

> This isn't a bug in the core networking code, but the addr parameter
> appears to be ignored here.

In mctp_test_create_dev_with_addr(), we are ignoring the addr argument
and just using `8`. Use the passed address instead.

All invocations use 8 anyway, so no effective change at present.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Simon Horman <horms@verge.net.au>
Link: https://patch.msgid.link/20260403-dev-mctp-fix-test-addr-v1-1-b7fa789cdd9b@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>