USB: fix concurrent buffer access in the hub driver

This patch (as849) fixes a bug in the USB hub driver.  A single
pre-allocated buffer is used for all port status reads, but nothing
guarantees exclusive use of the buffer.  A mutex is added to provide
this guarantee.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>