MIPS: ath79: use dynamically allocated USB platform devices

The current code uses static resources and static platform
device instances for the possible USB controllers in the
system. These static variables contains initial values which
leads to data segment pollution.

Remove the static variables and use dynamically allocated
structures instead.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4933/
Signed-off-by: John Crispin <blogic@openwrt.org>