usb: fix port reset

usb: fix port reset

commit 891fb2cd4592b6fe76106a69e0ca40efbf82726a removed the implicit
detach before (re-)attaching in usb_attach().  Some usb host controllers
used that behavior though to do a port reset by a detach+attach
sequence.

This patch establishes old behavior by adding a new usb_reset() function
for port resets and putting it into use, thereby also unifying port
reset behavior of all host controllers.  The patch also adds asserts to
usb_attach() and usb_detach() to make sure the calls are symmetrical.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>