sfc: Fix polling for slow MCDI operations

When the interface is down and we are using polled mode for MCDI
operations, we busy-wait for completion for approximately 1 jiffy
using udelay() and then back off to schedule().  But the completion
will not wake the task, since we are using polled mode!  We must use
schedule_timeout_uninterruptible() instead.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>