usb: musb: core: add pm_runtime_irq_safe()

We need a pm_runtime_get_sync() call from
within musb_gadget_pullup() to make sure
registers are accessible at that time.

The problem is that musb_gadget_pullup() is
called with IRQs disabled and, because of that,
we need to tell pm_runtime that this pm_runtime_get_sync()
is IRQ safe.

We can simply add pm_runtime_irq_safe(), however, because
we need to make our read/write accessor function pointers
have been initialized before trying to use them. This means
that all pm_runtime initialization for musb_core needs to
be moved down so that when we call pm_runtime_irq_safe(),
the pm_runtime_get_sync() that it calls on the parent, won't
cause a crash due to NULL musb_read/write accessors.

Reported-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>