USB: protect hcd.h from multiple inclusions

This will let us use this header in other header files.
Will be needed for the FHCI USB Host driver.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>