sfc: Serialise tenxpress_special_reset() with statistics fetches

On some boards 10Xpress feeds a 156 MHz clock to the Falcon XMAC.  MAC
statistics DMA can fail while this clock is stopped during a PHY reset.

From: Steve Hodgson <shodgson@solarflare.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>