musb: Avoid musb_gadget_pullup "Unhandled fault" oops on omap4

I've recently run across an "Unhandled fault: imprecise external abort"
oops that is caused when a driver called usb_gadget_connect() when there
was no cable plugged into the musb gadget port.

You can see the oops message here:
https://launchpadlibrarian.net/75635123/minicom.txt

Doing some digging, it seemed the problem was triggered when reading
from the musb registers in musb_pullup() when the device controller is
powered down.

Looking at other examples of where the registers were accessed, I
noticed they were always enclosed by pm_runtime_get/put calls. So I
added such calls to the musb_gadget_pullup() function and it seemed to
resolve the problem.

Now, full disclosure: this was triggered with the out-of-tree Android
adb gadget driver. However, I suspect the same behavior could be
triggered using the composite gadget driver as well, so I think this is
a generic issue. However, if I'm wrong, let me know and I'll try to make
sure the fix is done in the right place.

If this is the right fix, it probably should be queued for 3.1 and
3.0-stable.

Comments and feedback would be greatly appreciated!

Reported-by: Zach Pfeffer <zach.pfeffer@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>

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