USB: EHCI: ehci-mv: use a unique bus name

In case there are multiple Marvell EHCI blocks in system, we need a
different bus name for each one. Otherwise debugfs gets mildly upset about
a directory name in usb/ehci:

  debugfs: Directory 'mv ehci' with parent 'ehci' already present!

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20200309130014.548168-2-lkundrak@v3.sk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>