sfc: Remove EFX_WORKAROUND_9141 macro

The "bug9141 workaround" of setting TX_FLUSH_MIN_LEN_EN should really
be considered as a normal bit of configuration rather than a
workaround.

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