ar71xx: don't register USB device on WNDAP360

Apparently registering usb device on newest trunk causes
this AP not to boot...
Since there is no USB socket in that device anyway this
can be simply removed (and device boots then no problem).

[juhosg:
  - tweak subject line and description,
  - remove #include "dev-usb.h",
  - remove 'select ATH79_DEV_USB' from Kconfig]

Signed-off-by: Jacek Kikiewicz <jaceq@aol.pl>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 37844