Simplify the handling of small packets padding in cpsw:

• Pad small packets to 60 bytes and not 64 (exclude the CRC bytes);
• Pad the packet using m_append(9), if the packet has enough space for padding, which is usually true, it will not be necessary append a newly allocated mbuf to the chain.

Suggested by: yongari
MFC after: 3 days
Sponsored by: Rubicon Communications, LLC (Netgate)