sfc: Rename 'xfp' file and functions to reflect reality

The 'XFP' driver is really a driver for the QT2022C2 and QT2025C PHYs,
covering both more and less than XFP.  Rename its functions and
constants to reflect reality and to reduce namespace pollution when
sfc is a built-in driver.

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