MIPS: ath79: use a helper function for USB resource initialization

This improves code readability, and ensures that
all resource fields will be initialized correctly.
Additionally, it helps to reduce the size of the
kernel image by using uninitialized resource
variables.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4171/
Signed-off-by: John Crispin <blogic@openwrt.org>