ehci-hcd: fix memory leak in lowlevel init

usb_lowlevel_init() allocates a new periodic_list each time it is invoked,
without freeing the original list. Since it is initialized later on in the code,
just reuse the first-allocated list in future invocations of usb_lowlevel_init.

Cc: Marek Vasut <marex@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>