sfc: Read MC firmware version when requested through ethtool

We currently make no use of siena_nic_data::fw_{version,build} except
to format the firmware version for ethtool_get_drvinfo().  Since we
only read the version at start of day, this information is incorrect
after an MC firmware update.  Remove the cached version information
and read it via MCDI whenever it is requested.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>