USB: musb: do not proceed with interrupt completions if qh is not ready

It's not safe to modify hw_ep if qh is not ready.
This may corrupt data in musb_qh. In RX/TX completions this
may happen on SMP system with current musb host implementation
The main issue is with musb_giveback function. It unlocks and
and after that locks musb->lock so during this time
musb_qh data may be corrupted. This results in null-pointers
dereferences / programming of busy DMA channel / etc in
different places.

This often happens during unlinking URB for ISO and looks like:
CPU0                                  CPU1
...                                   musb_urb_dequeue
...                                      spin_lock_irqsave(&musb->lock)
...                                      (work with musb_qh)
...                                      musb_giveback
...                                         spin_unlock(&musb->lock)
generic_interrupt()                         ...
   spin_lock_irqsave(&musb->lock)           ...
   (work with musb_qh)                      ...
   spin_unlock_irqrestore(&musb->lock)      ...
...                                         spin_lock(&musb->lock)
...                                      (continues work with musb_qh)
...                                      spin_lock_irqrestore(&musb->lock)

Go out from completion if qh is not ready.

Change-Id: I9c9280a7ab7f8e89b2e8bf745800217f903108fc
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>
(cherry picked from commit d0d4f29c2ac05a66a8b26c00ddd436e71519a1d5)