USB: Drop cache flush bloat in EHCI-HCD

USB: Drop cache flush bloat in EHCI-HCD

Discard the creepy cache flushing mechanisms in ehci-hcd.c and replace them with
more straightforward flushing. In the new approach, the flushing takes place
directly in ehci_submit_async() call instead of going through the QH list and
flushing all members and buffers. This discards a lot of weird bit operations
on the members of QH and qTD structures.

NOTE: Certainly, this flushes even qTDs which are possibly unused in some
transactions, though the overhead of the previous code was much higher than is
the overhead of flushing two more cache lines (which most probably aren't even
cached).

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>