sfc: correctly initialise filter rwsem for farch

Fixes: fc7a6c28 ("sfc: use a semaphore to lock farch filters too")
Suggested-by: Joseph Korty <joe.korty@concurrent-rt.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>