sfc: Avoid generating over-length MC_CMD_FLUSH_RX_QUEUES request

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>