selftests: drv-net: rss: Generate unique ports for RSS context tests

selftests: drv-net: rss: Generate unique ports for RSS context tests

The RSS ctx tests rely on NFC rules with unique ports to steer packets
to the correct ctx. This updates the test to use the new rand_ports()
helper to guarantee the ports are unique.

Manual testing shows that generating 32 ports with the existing method
would result in at least one duplicate 4% of the time.

Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Link: https://patch.msgid.link/20260224224659.1507082-3-dimitri.daskalakis1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>