sfc: move NIC-specific mcdi_port declarations out of common header

These functions are implemented in mcdi_port.c, which will not be linked
into the EF100 driver; thus their prototypes should not be visible in
common header files.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>