usb: ehci: do not invalidate a NULL buffer

usb: ehci: do not invalidate a NULL buffer

Its a valid use case to call ehci_submit_async() with a NULL buffer
with length 0. E.g. from usb_set_configuration().

As invalidate_dcache_range() isn't able to judge if the address
NULL is valid or not (depending on the SoC hardware configuration it
might be valid) do the check in ehci_submit_async() as here we know
that we don't have to invalidate such a buffer.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>