usb: musb: Use delayed for musb_gadget_pullup

We have MUSB setting pm_runtime_irq_safe with the following
commits:

30a70b02 ("usb: musb: fix obex in g_nokia.ko causing kernel panic")
3e43a072 ("usb: musb: core: add pm_runtime_irq_safe()")

Let's fix things to use delayed work so we can remove the
pm_runtime_irq_safe.

Note that we may want to set this up in a generic way in the
gadget framework eventually.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>