sfc: Fix type of FALCON_SPI_MAX_LEN

FALCON_SPI_MAX_LEN has type size_t while other SPI lengths have type
unsigned int.  This results in warnings from min() on 64-bit
architectures where they are different.  Add a cast to make it match.

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