tests: net: wifi: exclude RW612 ethernet variant

Ethernet board variant was missed in commit 138e9591 (tests: net:
wifi: exclude RW612 based boards), but is the sole variant that actually
was running this test. Exclude this variant only, since it cannot build
this test due to requiring binary blobs.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>