selftests: drv-net: fix RPS mask handling for high CPU numbers

selftests: drv-net: fix RPS mask handling for high CPU numbers

The RPS bitmask bounds check uses ~(RPS_MAX_CPUS - 1) which equals ~15 =
0xfff0, only allowing CPUs 0-3.

Change the mask to ~((1UL << RPS_MAX_CPUS) - 1) = ~0xffff to allow CPUs
0-15.

Fixes: 5ebfb4cc3048 ("selftests/net: toeplitz test")
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260112173715.384843-3-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>