sfc: Don't use enums as a bitmask.

This fixes:

drivers/net/sfc/mcdi_mac.c: In function ‘efx_mcdi_set_mac’:
drivers/net/sfc/mcdi_mac.c:36:2: warning: case value ‘3’ not in enumerated type ‘enum efx_fc_type’

Signed-off-by: David S. Miller <davem@davemloft.net>