USB: ehci: fix remove of ehci debugfs dir

The patch below on gregkh tree only creates 'lpm' file under
ehci->debug_dir, but not removes it when unloading module,

	 USB: EHCI: EHCI 1.1 addendum: preparation

which can make loading of ehci-hcd module failed after unloading it.

This patch replaces debugfs_remove with debugfs_remove_recursive
to remove ehci debugfs dir and files. It does fix the bug above,
and may simplify the removing procedure.

Also, remove the debug_registers, debug_async and debug_periodic
field from ehci_hcd struct since they are useless now.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>