usb: host: ehci-sched: add comment about find_tt() not returning error

Add a comment explaining why find_tt() will not return error even though
find_tt() is checking for NULL and other errors.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Link: https://lore.kernel.org/r/20201011205008.24369-1-sudipm.mukherjee@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>