sfc: Demote "MC Scheduler error" messages

The MC firmware is cooperatively multitasking and its scheduler will
send an event when a task yields after running for more than the
expected maximum time.  This can be useful for firmware development
but does not usually indicate a serious error and does not help to
detect a lockup (there is a hardware watchdog that does that).
Change the message and reduce log level accordingly.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>