am57xx: Remove unused variable warnings

Starting with the changes to fix USB host on am57xx/am43xx we stopped
using usb_otg_ss1/related stuff and but we hadn't been enabling the
relevant options to cause the warnings until just recently.

Fixes: 55efadde (ARM: AM57xx: AM43xx: Fix USB host)
Fixes: a48d687c (configs: am57xx: Enable download gadget)
Signed-off-by: Tom Rini <trini@konsulko.com>