tlan: Fix small (< 64 bytes) datagram transmissions

The TLAN chip does not support tranmissions smaller than 64
bytes. Smaller transfers need to be padded up to that size. This was
broken by commit id 41873e9aff0632d80c74380d58a89e8d420151bd ("tlan:
get rid of padding buffer").

<URL:http://bugzilla.kernel.org/show_bug.cgi?id=11754>

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>