usb driver for intellon int51x1 based PLC like devolo dlan duo

usb driver for intellon int51x1 based PLC like devolo dlan duo
with improvements suggested by the guys of the mailinglist:
- name and prefix with int51x1 (Florian Fainelli)
- use conversion functions cpu_to_le16 / le16_to_cpu (Oliver Neukum)
- use pskb_may_pull instead of skb->len (Ilpo Järvinen)
- better code in tx_fixup (Ilpo Järvinen)
- use gotos for error handling (Ilpo Järvinen)
- better description (Jon Smirl)

Signed-off-by: Peter Holik <peter@holik.at>
Signed-off-by: David S. Miller <davem@davemloft.net>