USB: use generic platform driver on ath79

The ath79 usb driver doesn't do anything special and is now converted
to the generic ehci and ohci driver.
This was tested on a TP-Link TL-WR1043ND (AR9132)

Acked-by: Gabor Juhos <juhosg@openwrt.org>
CC: Imre Kaloz <kaloz@openwrt.org>
CC: linux-mips@linux-mips.org
CC: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>