sfc: Use TX PIO for sufficiently small packets

Sufficiently small linear packets can be copied into the PIO buffer
with a single call to memcpy_toio().  Non-linear packets require an
intermediate cache-line-sized buffer.

[bwh: I wrote the first version of this, but Jon did the hard work to
 handle non-linear packets.]
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>