sfc: Make board information explicitly Falcon-specific

Rename struct efx_board to struct falcon_board.

Introduce and use inline function to look up board info from struct
efx_nic, in preparation for moving it.

Move board init and fini calls into NIC probe and remove functions.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>