sfc: move datapath management code

The code that manages the datapath (starting, stopping, including the
port-related bits) will be common.
Three functions have been added that contain bits from other
functions. These will be moved to their final files in later patches.
Small code styling fixes included.

Signed-off-by: Alexandru-Mihai Maftei <amaftei@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>