MCDI supports requests up to 252 bytes long, which is only enough to pass 63 RX queue IDs to MC_CMD_FLUSH_RX_QUEUES. However a VF may have up to 64 RX queues, and if we try to flush them all we will generate an over-length request and BUG() in efx_mcdi_copyin(). Currently all VF drivers limit themselves to 32 RX queues, so reducing the limit to 63 does no harm. Also add a BUILD_BUG_ON in efx_mcdi_flush_rxqs() so we remember to deal with the same problem there if EFX_MAX_CHANNELS is increased. Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>