usb: musb: move work_struct(otg_notifier_work) from core to omap glue

Commit 712d8e(fixes pm_runtime calls while atomic by using a work
queue. musb pm_runtime_get_sync call happens in interrupt context
on cable attach case. That can result in re-enabling the interrupts and
cause side affect. To avoid this deferred processing is used)

While the issue and the work queue implementation is specific to omap
(omap2430.c), the work_struct is defined as a member of struct musb
(musb_core.h). Hence moved the work_struct from musb_core to omap
glue.

Cc: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>