USB: Fix doc for usb_autopm_enable

Correct errors in the descriptions for usb_autopm_enable
and usb_autopm_disable in the USB PM doc.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>