sfc: return a better error if filter insertion collides with MC reboot

If some other operation gets the MCDI lock ahead of us and performs an MC
 reboot, then our attempt to insert the filter will fail with EINVAL,
 because the destination VI (spec->dmaq_id, MC_CMD_FILTER_OP_IN_RX_QUEUE) does
 not exist.  But the caller's request (which might e.g. be an ethtool ntuple
 request from userland) isn't invalid, it just got unlucky; so return EAGAIN.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>