sfc: split up nic.h

The new nic_common.h contains the inlines for NIC-type function dispatch,
 declarations for NIC-generic functions in nic.c, and other similar NIC-
 generic functionality.  Retained in nic.h are NIC-specific declarations
 such as the siena and ef10 nic_data structs and various farch functions.

The EF100 driver will thus include nic_common.h but not nic.h.

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