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>
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