usb: ohci-spear: Remove direct access to platform_device resource list

The memory resource is already retrieved with platform_get_resource(), so
let's use it instead of assuming it is the first resource in the array.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211215225509.1995417-1-robh@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>