ethtool: Null-terminate filename passed to ethtool_ops::flash_device

The parameters for ETHTOOL_FLASHDEV include a filename, which ought to
be null-terminated.  Currently the only driver that implements
ethtool_ops::flash_device attempts to add a null terminator if
necessary, but does it wrongly.  Do it in the ethtool core instead.

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