usb: musb: Fix external abort on non-linefetch for musb_irq_work()

While testing musb host mode cable plugging on a BeagleBone, I came across this
error:

Unhandled fault: external abort on non-linefetch (0x1008) at 0xd1dcfc60
...
[<bf668390>] (musb_default_readb [musb_hdrc]) from [<bf668578>] (musb_irq_work+0x1c/0x180 [musb_hdrc])
[<bf668578>] (musb_irq_work [musb_hdrc]) from [<c0156554>] (process_one_work+0x2b4/0x808)
[<c0156554>] (process_one_work) from [<c015767c>] (worker_thread+0x3c/0x550)
[<c015767c>] (worker_thread) from [<c015d568>] (kthread+0x104/0x148)
[<c015d568>] (kthread) from [<c01078d0>] (ret_from_fork+0x14/0x24)

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>