eth: fbnic: fix typo in compile assert

We should be validating the Rx count on the Rx struct,
not the Tx struct. There is no real change here, rx_stats
and tx_stats are instances of the same struct.

Acked-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250306145150.1757263-3-kuba@kernel.org


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