CONFIG_ATH79_DEV_USB is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_ATH79_DEV_USB (not configurable)

modulename: dev-usb.ko

configname: CONFIG_ATH79_DEV_USB

Linux Kernel Configuration
└─>ATH79_DEV_USB
In linux kernel since version 3.1 (release Date: 2011-10-24)  
source code:
arch/mips/ath79/dev-usb.c
depends
CONFIG_ATH79

is depended by
CONFIG_ATH79_MACH_AP136